diff --git a/ChangeLog b/ChangeLog
index c14fed5908..548eaddfa4 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Thu Dec 13 14:02:15 2012  NAKAMURA Usaku  <usa@ruby-lang.org>
+
+	* file.c (rb_file_join): check encoding compatibility before joining
+	  strings.
+
 Thu Dec 13 13:06:27 2012  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* proc.c (umethod_bind): allow another form of method transplanting
diff --git a/file.c b/file.c
index 88a7065cf7..2aa896ae47 100644
--- a/file.c
+++ b/file.c
@@ -3940,6 +3940,7 @@ rb_file_join(VALUE ary, VALUE sep)
     VALUE result, tmp;
     const char *name, *tail;
     int checked = TRUE;
+    rb_encoding *enc;
 
     if (RARRAY_LEN(ary) == 0) return rb_str_new(0, 0);
 
@@ -3994,10 +3995,14 @@ rb_file_join(VALUE ary, VALUE sep)
 		rb_str_set_len(result, tail - name);
 	    }
 	    else if (!*tail) {
+		enc = rb_enc_check(result, sep);
 		rb_str_buf_append(result, sep);
+		rb_enc_associate(result, enc);
 	    }
 	}
+	enc = rb_enc_check(result, tmp);
 	rb_str_buf_append(result, tmp);
+	rb_enc_associate(result, enc);
     }
     RBASIC(result)->klass = rb_cString;