diff --git a/README.md b/README.md
index c3c0a53..ec5a6dc 100644
--- a/README.md
+++ b/README.md
@@ -53,7 +53,7 @@ This project is built on work of [jb](https://github.com/amatsuda/jb).
 
 ## Template Syntax
 
-SimpleJson templates are simply lambda objects that return data(Hashes or Arrays) for json.
+SimpleJson templates should be written in Ruby lambda format. The template code is converted into a method and then invoked to produce data (Hashes or Arrays) for JSON.
 
 ```ruby
 -> {
diff --git a/lib/simple_json/simple_json_renderer.rb b/lib/simple_json/simple_json_renderer.rb
index ba4e1a3..5961765 100644
--- a/lib/simple_json/simple_json_renderer.rb
+++ b/lib/simple_json/simple_json_renderer.rb
@@ -20,7 +20,7 @@ def load_all_templates!
           template_files = Rails.root.glob("#{path}/**/*.simple_json.rb")
           template_files.each do |file_path|
             template_path = file_path.relative_path_from(Rails.root.join(path)).to_path.delete_suffix('.simple_json.rb')
-            @renderers[template_path] = SimpleJsonTemplate.new(file_path.to_path).renderer
+            define_template_method(template_path, file_path.to_path)
           end
         end
         @templates_loaded = true
@@ -29,7 +29,7 @@ def load_all_templates!
       def load_template(template_path)
         if SimpleJson.template_cache_enabled?
           load_all_templates! unless templates_loaded?
-          renderers[template_path]
+          render_methods[template_path]
         else
           load_template_from_file(template_path)
         end
@@ -38,18 +38,36 @@ def load_template(template_path)
       def load_template_from_file(template_path)
         SimpleJson.template_paths.each do |path|
           file_path = Rails.root.join("#{path}/#{template_path}.simple_json.rb").to_path
-          return SimpleJsonTemplate.new(file_path).renderer if File.exist?(file_path)
+
+          return define_template_method(template_path, file_path) if File.exist?(file_path)
         end
 
         nil
       end
 
-      def renderers
-        @renderers ||= {}
+      def define_template_method(template_path, file_path)
+        template = SimpleJsonTemplate.new(file_path)
+        code_hash = template.code.hash
+
+        render_methods[template_path] = render_methods_cache.fetch(code_hash) do
+          @template_num ||= 0
+          @template_num += 1
+          method_name = :"template_#{@template_num}"
+          template.define_to_class(self, method_name)
+          render_methods_cache[code_hash] = method_name
+        end
+      end
+
+      def render_methods
+        @render_methods ||= {}
+      end
+
+      def render_methods_cache
+        @render_methods_cache ||= {}
       end
 
       def clear_renderers
-        @renderers = {}
+        @render_methods = {}
         @templates_loaded = false
       end
     end
@@ -70,10 +88,11 @@ def renderers
     end
 
     def render(template_name, **params)
+      method_name = renderer(template_name)
       if !params.empty?
-        instance_exec(**params, &renderer(template_name))
+        send(method_name, **params)
       else
-        instance_exec(&renderer(template_name))
+        send(method_name)
       end
     end
 
diff --git a/lib/simple_json/simple_json_template.rb b/lib/simple_json/simple_json_template.rb
index deab7b3..a375e32 100644
--- a/lib/simple_json/simple_json_template.rb
+++ b/lib/simple_json/simple_json_template.rb
@@ -7,8 +7,11 @@ def initialize(path)
       @source = File.read(path)
     end
 
-    def renderer
-      @renderer ||= eval(code, TOPLEVEL_BINDING, @path) # rubocop:disable Security/Eval
+    def define_to_class(klass, method_name)
+      method_string = to_method_string(method_name)
+      klass.class_eval(method_string, @path)
+
+      method_name
     end
 
     def code
@@ -22,5 +25,27 @@ def lambda_stringify(source)
 
       "-> { #{source} }"
     end
+
+    def method_string_from_lambda(source, method_name)
+      pattern = %r{
+        \A(?<prefix>(?:\s*\#.*\n|\s+)*)
+        ->\s*
+        (?:\((?<params_p>.*?)\)|(?<params_n>[^\{ ]*?))
+        \s*(?:\{(?<body>.*)\}|do(?<body>.*)end)
+        (?<suffix>(?:\s*\#.*|\s+)*)\z
+      }mx
+
+      match = source.match(pattern)
+      raise :parse_error unless match
+
+      params = (match[:params_p] || match[:params_n] || "").strip
+      arg_part = params.empty? ? "" : "(#{params})"
+
+      "#{match[:prefix]}def #{method_name}#{arg_part};#{match[:body]};end#{match[:suffix]}"
+    end
+
+    def to_method_string(method_name)
+      method_string_from_lambda(code, method_name)
+    end
   end
 end
diff --git a/test/unit/simple_json_template_test.rb b/test/unit/simple_json_template_test.rb
new file mode 100644
index 0000000..6d6b837
--- /dev/null
+++ b/test/unit/simple_json_template_test.rb
@@ -0,0 +1,251 @@
+# frozen_string_literal: true
+
+require 'test_helper'
+
+class SimpleJsonTemplateTest < Test::Unit::TestCase
+  # -> { body } form
+  test 'converts simple lambda with braces' do
+    source = '-> { "hello" }'
+    result = call_method(source, :foo)
+
+    assert_equal 'def foo; "hello" ;end', result
+  end
+
+  # ->(params) { body } form (parenthesized parameters)
+  test 'converts lambda with parenthesized parameters' do
+    source = '->(a, b) { a + b }'
+    result = call_method(source, :add)
+
+    assert_equal 'def add(a, b); a + b ;end', result
+  end
+
+  # ->params { body } form (no parentheses, no space)
+  test 'converts lambda with parameter without parentheses' do
+    source = '->a { a * 2 }'
+    result = call_method(source, :double)
+
+    assert_equal 'def double(a); a * 2 ;end', result
+  end
+
+  # -> do body end form
+  test 'converts lambda with do-end' do
+    source = '-> do "hello" end'
+    result = call_method(source, :foo)
+
+    assert_equal 'def foo; "hello" ;end', result
+  end
+
+  # ->(params) do body end form
+  test 'converts lambda with parameters and do-end' do
+    source = '->(x) do x * 2 end'
+    result = call_method(source, :double)
+
+    assert_equal 'def double(x); x * 2 ;end', result
+  end
+
+  # Multiline body
+  test 'converts multiline lambda body' do
+    source = <<~RUBY
+      -> {
+        x = 1
+        y = 2
+        x + y
+      }
+    RUBY
+    result = call_method(source, :calc)
+
+    assert_match(/def calc;/, result)
+    assert_match(/x = 1/, result)
+    assert_match(/y = 2/, result)
+    assert_match(/x \+ y/, result)
+    assert_match(/;end/, result)
+  end
+
+  # Preserve prefix comments
+  test 'preserves prefix comments' do
+    source = <<~RUBY
+      # This is a comment
+      -> { "hello" }
+    RUBY
+    result = call_method(source, :foo)
+
+    assert_match(/\A# This is a comment\n/, result)
+    assert_match(/def foo;/, result)
+  end
+
+  # Preserve suffix comments
+  test 'preserves suffix comments' do
+    source = <<~RUBY
+      -> { "hello" }
+      # trailing comment
+    RUBY
+    result = call_method(source, :foo)
+
+    assert_match(/;end\n# trailing comment/, result)
+  end
+
+  # Empty parameter list
+  test 'converts lambda with empty parentheses' do
+    source = '->() { "hello" }'
+    result = call_method(source, :foo)
+
+    assert_equal 'def foo; "hello" ;end', result
+  end
+
+  # Space before brace with no parameters
+  test 'converts lambda with space before brace' do
+    source = '->   { "hello" }'
+    result = call_method(source, :foo)
+
+    assert_equal 'def foo; "hello" ;end', result
+  end
+
+  # Multiline do-end form
+  test 'converts multiline do-end lambda' do
+    source = <<~RUBY
+      -> do
+        result = []
+        result << 1
+        result
+      end
+    RUBY
+    result = call_method(source, :build)
+
+    assert_match(/def build;/, result)
+    assert_match(/result = \[\]/, result)
+    assert_match(/;end/, result)
+  end
+
+  # --- Parameter types ---
+
+  # Default argument
+  test 'converts lambda with default parameter' do
+    source = '->(a, b = 1) { a + b }'
+    result = call_method(source, :add)
+
+    assert_equal 'def add(a, b = 1); a + b ;end', result
+  end
+
+  # Keyword arguments
+  test 'converts lambda with keyword parameters' do
+    source = '->(a:, b: 1) { a + b }'
+    result = call_method(source, :add)
+
+    assert_equal 'def add(a:, b: 1); a + b ;end', result
+  end
+
+  # Variadic arguments
+  test 'converts lambda with splat parameter' do
+    source = '->(*args) { args }'
+    result = call_method(source, :collect)
+
+    assert_equal 'def collect(*args); args ;end', result
+  end
+
+  # Block parameter
+  test 'converts lambda with block parameter' do
+    source = '->(&block) { block.call }'
+    result = call_method(source, :execute)
+
+    assert_equal 'def execute(&block); block.call ;end', result
+  end
+
+  # Double splat
+  test 'converts lambda with double splat parameter' do
+    source = '->(**opts) { opts }'
+    result = call_method(source, :options)
+
+    assert_equal 'def options(**opts); opts ;end', result
+  end
+
+  # --- Body edge cases ---
+
+  # Nested braces
+  test 'converts lambda with nested braces in body' do
+    source = '-> { { key: value } }'
+    result = call_method(source, :hash)
+
+    assert_equal 'def hash; { key: value } ;end', result
+  end
+
+  # Empty body (braces)
+  test 'converts lambda with empty body' do
+    source = '-> { }'
+    result = call_method(source, :noop)
+
+    assert_equal 'def noop; ;end', result
+  end
+
+  # Empty body (do-end)
+  test 'converts do-end lambda with empty body' do
+    source = '-> do end'
+    result = call_method(source, :noop)
+
+    assert_equal 'def noop; ;end', result
+  end
+
+  # do-end body contains end keyword
+  test 'converts do-end lambda with nested end keyword' do
+    source = '-> do if x then y end end'
+    result = call_method(source, :conditional)
+
+    assert_equal 'def conditional; if x then y end ;end', result
+  end
+
+  # Body string contains end
+  test 'converts lambda with end string in body' do
+    source = '-> { "end of string" }'
+    result = call_method(source, :message)
+
+    assert_equal 'def message; "end of string" ;end', result
+  end
+
+  # --- Prefix edge cases ---
+
+  # Multiple prefix comments
+  test 'preserves multiple prefix comments' do
+    source = <<~RUBY
+      # comment 1
+      # comment 2
+      -> { 1 }
+    RUBY
+    result = call_method(source, :foo)
+
+    assert_match(/\A# comment 1\n# comment 2\n/, result)
+    assert_match(/def foo;/, result)
+  end
+
+  # Blank line prefix
+  test 'preserves blank line prefix' do
+    source = "\n\n-> { 1 }"
+    result = call_method(source, :foo)
+
+    assert_match(/\A\n\ndef foo;/, result)
+  end
+
+  # --- Error cases ---
+
+  # Non-lambda input
+  test 'raises error for non-lambda input' do
+    source = 'def foo; end'
+
+    assert_raise(TypeError) do
+      call_method(source, :foo)
+    end
+  end
+
+  # Proc.new input
+  test 'raises error for Proc.new input' do
+    source = 'Proc.new { 1 }'
+
+    assert_raise(TypeError) do
+      call_method(source, :foo)
+    end
+  end
+
+  private
+
+  def call_method(source, method_name)
+    SimpleJson::SimpleJsonTemplate.allocate.send(:method_string_from_lambda, source, method_name)
+  end
+end