diff --git a/docs/lang.rst b/docs/lang.rst
index 6a68fed7..9c4152ff 100644
--- a/docs/lang.rst
+++ b/docs/lang.rst
@@ -58,7 +58,7 @@ section                                          status     comments
 5.3.1 Top level definitions                      yes
 5.3.2 Internal definitions                       yes        TODO: interreferential definitions
 5.3.3 Multiple-value definitions                 yes
-5.4 Syntax definitions                           yes        TODO: internal macro definition is not supported.
+5.4 Syntax definitions                           yes
 5.5 Recored-type definitions                     yes
 5.6.1 Library Syntax                             incomplete In picrin, libraries can be reopend and can be nested.
 5.6.2 Library example                            N/A
diff --git a/piclib/built-in.scm b/piclib/built-in.scm
index 8221653e..c3c09059 100644
--- a/piclib/built-in.scm
+++ b/piclib/built-in.scm
@@ -292,6 +292,17 @@
                           `(,(r 'begin) ,@(cdar clauses)))
                         ,(loop (cdr clauses))))))))))
 
+  (define-syntax letrec-syntax
+    (er-macro-transformer
+     (lambda (form r c)
+       (let ((formal (car (cdr form)))
+             (body   (cdr (cdr form))))
+         `(let ()
+            ,@(map (lambda (x)
+                     `(,(r 'define-syntax) ,(car x) ,(cadr x)))
+                   formal)
+            ,@body)))))
+
   (define-syntax syntax-error
     (er-macro-transformer
      (lambda (expr rename compare)
@@ -317,6 +328,7 @@
           and or
           cond case else =>
           do when unless
+          letrec-syntax
           _ ... syntax-error))
 
 
@@ -629,6 +641,7 @@
         and or
         cond case else =>
         do when unless
+        letrec-syntax
         _ ... syntax-error)
 
 (export let-values