diff --git a/ChangeLog b/ChangeLog
index b6bb4db562..3311c3dfae 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Mon Oct  5 00:53:51 2015  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* dir.c (rb_dir_getwd): make ASCII-8BIT if filesystem encoding is
+	  US-ASCII, like as Dir.glob.
+
 Sun Oct  4 23:39:09 2015  Tanaka Akira  <akr@fsij.org>
 
 	* enum.c (nmin_filter): Fix limit value.
diff --git a/dir.c b/dir.c
index d3f6530663..36cdadc0e9 100644
--- a/dir.c
+++ b/dir.c
@@ -1010,7 +1010,9 @@ rb_dir_getwd(void)
 {
     char *path;
     VALUE cwd;
+    int fsenc = rb_enc_to_index(rb_filesystem_encoding());
 
+    if (fsenc == ENCINDEX_US_ASCII) fsenc = ENCINDEX_ASCII;
     path = my_getcwd();
 #ifdef __APPLE__
     cwd = rb_str_normalize_ospath(path, strlen(path));
@@ -1018,7 +1020,7 @@ rb_dir_getwd(void)
 #else
     cwd = rb_tainted_str_new2(path);
 #endif
-    rb_enc_associate(cwd, rb_filesystem_encoding());
+    rb_enc_associate_index(cwd, fsenc);
 
     xfree(path);
     return cwd;