diff --git a/ChangeLog b/ChangeLog
index c48dfc06ed..60643cfb43 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Fri Feb 16 19:24:22 2007  Yukihiro Matsumoto  <matz@ruby-lang.org>
+
+	* math.c (math_log): update document to mention second optional
+	  argument for logarithm base. 
+
 Fri Feb 16 19:19:21 2007  Yukihiro Matsumoto  <matz@ruby-lang.org>
 
 	* parse.y (mrhs): need to append by arg_append().
diff --git a/math.c b/math.c
index 2675c71e6d..bd02849002 100644
--- a/math.c
+++ b/math.c
@@ -305,8 +305,11 @@ math_exp(VALUE obj, VALUE x)
 /*
  *  call-seq:
  *     Math.log(numeric)    => float
+ *     Math.log(num,base)   => float
  *  
  *  Returns the natural logarithm of <i>numeric</i>.
+ *  If additional second argument is given, it will be the base
+ *  of logarithm.
  */
 
 static VALUE