diff --git a/ChangeLog b/ChangeLog
index 1a7df151a5..b03dcfc33a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,4 +1,6 @@
-Sat Aug 14 15:17:36 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+Sat Aug 14 15:19:55 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* io.c (rb_io_memsize): constified.
 
 	* regcomp.c (onig_memsize): constified.
 
diff --git a/ext/objspace/objspace.c b/ext/objspace/objspace.c
index b557cc9ab3..65fa201d46 100644
--- a/ext/objspace/objspace.c
+++ b/ext/objspace/objspace.c
@@ -33,7 +33,7 @@
 
 size_t rb_str_memsize(VALUE);
 size_t rb_ary_memsize(VALUE);
-size_t rb_io_memsize(rb_io_t *);
+size_t rb_io_memsize(const rb_io_t *);
 size_t rb_generic_ivar_memsize(VALUE);
 size_t rb_objspace_data_type_memsize(VALUE obj);
 
diff --git a/io.c b/io.c
index e92c88117a..8fbb6cbc99 100644
--- a/io.c
+++ b/io.c
@@ -3536,7 +3536,7 @@ rb_io_fptr_finalize(rb_io_t *fptr)
 size_t rb_econv_memsize(rb_econv_t *);
 
 size_t
-rb_io_memsize(rb_io_t *fptr)
+rb_io_memsize(const rb_io_t *fptr)
 {
     size_t size = sizeof(rb_io_t);
     size += fptr->rbuf_capa;