diff --git a/doc/extension.ja.rdoc b/doc/extension.ja.rdoc
index 30f1b566d332e5..4f79186f926393 100644
--- a/doc/extension.ja.rdoc
+++ b/doc/extension.ja.rdoc
@@ -226,6 +226,10 @@ rb_tainted_str_new_cstr(const char *ptr) ::
 
   Cの文字列から汚染マークが付加されたRubyの文字列を生成する．
 
+rb_str_append(VALUE str1, VALUE str2) ::
+
+  Rubyの文字列str1にRubyの文字列str2を追加する．
+
 rb_sprintf(const char *format, ...) ::
 rb_vsprintf(const char *format, va_list ap) ::
 
@@ -251,8 +255,8 @@ rb_str_vcatf(VALUE str, const char* format, va_list ap) ::
 
   Cの文字列formatと続く引数をprintf(3)のフォーマットにしたがって
   整形し，Rubyの文字列strに追加する．この関数の機能は，それぞれ
-  rb_str_cat2(str, rb_sprintf(format, ...)) や
-  rb_str_cat2(str, rb_vsprintf(format, ap)) と同等である．
+  rb_str_append(str, rb_sprintf(format, ...)) や
+  rb_str_append(str, rb_vsprintf(format, ap)) と同等である．
 
 rb_enc_str_new(const char *ptr, long len, rb_encoding *enc) ::
 rb_enc_str_new_cstr(const char *ptr, rb_encoding *enc) ::
diff --git a/doc/extension.rdoc b/doc/extension.rdoc
index 83be591aa7a6b0..167635c6290e24 100644
--- a/doc/extension.rdoc
+++ b/doc/extension.rdoc
@@ -210,6 +210,10 @@ rb_vsprintf(const char *format, va_list ap) ::
   must be a VALUE).  Since it conflicts with "%i", for integers in
   format strings, use "%d".
 
+rb_str_append(VALUE str1, VALUE str2) ::
+
+  Appends str2 Ruby string to str1 Ruby string.
+
 rb_str_cat(VALUE str, const char *ptr, long len) ::
 
   Appends len bytes of data from ptr to the Ruby string.
@@ -225,8 +229,8 @@ rb_str_vcatf(VALUE str, const char* format, va_list ap) ::
 
   Appends C string format and successive arguments to Ruby string
   str according to a printf-like format.  These functions are
-  equivalent to rb_str_cat2(str, rb_sprintf(format, ...)) and
-  rb_str_cat2(str, rb_vsprintf(format, ap)), respectively.
+  equivalent to rb_str_append(str, rb_sprintf(format, ...)) and
+  rb_str_append(str, rb_vsprintf(format, ap)), respectively.
 
 rb_enc_str_new(const char *ptr, long len, rb_encoding *enc) ::
 rb_enc_str_new_cstr(const char *ptr, rb_encoding *enc) ::