diff --git a/ble.pp b/ble.pp
index 376b665f..4b564c3f 100644
--- a/ble.pp
+++ b/ble.pp
@@ -877,14 +877,28 @@ function ble/base/initialize-base-directory {
 ##   3. $_ble_base/tmp/$UID を使う。
 ##
 function ble/base/initialize-runtime-directory/.xdg {
-  local runtime_dir=${XDG_RUNTIME_DIR:-/run/user/$UID}
-  if [[ ! -d $runtime_dir ]]; then
-    [[ $XDG_RUNTIME_DIR ]] &&
+  local runtime_dir=
+  if [[ $XDG_RUNTIME_DIR ]]; then
+    if [[ ! -d $XDG_RUNTIME_DIR ]]; then
       ble/util/print "ble.sh: XDG_RUNTIME_DIR='$XDG_RUNTIME_DIR' is not a directory." >&2
-    return 1
+      return 1
+    elif [[ -O $XDG_RUNTIME_DIR ]]; then
+      runtime_dir=$XDG_RUNTIME_DIR
+    else
+      # When XDG_RUNTIME_DIR is not owned by the current user, maybe "su" is
+      # used to enter this session keeping the environment variables of the
+      # original user.  We just ignore XDG_RUNTIME_DIR (without issueing
+      # warnings) for such a case.
+      false
+    fi
+  fi
+  if [[ ! $runtime_dir ]]; then
+    runtime_dir=/run/user/$UID
+    [[ -d $runtime_dir && -O $runtime_dir ]] || return 1
   fi
+
   if ! [[ -r $runtime_dir && -w $runtime_dir && -x $runtime_dir ]]; then
-    [[ $XDG_RUNTIME_DIR ]] &&
+    [[ $runtime_dir == "$XDG_RUNTIME_DIR" ]] &&
       ble/util/print "ble.sh: XDG_RUNTIME_DIR='$XDG_RUNTIME_DIR' doesn't have a proper permission." >&2
     return 1
   fi
diff --git a/docs/ChangeLog.md b/docs/ChangeLog.md
index 4064e1af..44a54b83 100644
--- a/docs/ChangeLog.md
+++ b/docs/ChangeLog.md
@@ -268,6 +268,7 @@
 - builtin: print usages of emulated builtins on option errors `#D1694` 6f74021
 - decode (`ble/builtin/bind`): improve compatibility of the deprecated form `bind key:rlfunc` (motivated by cmplstofB) `#D1698` b6fc4f0
 - complete: work around a false warning messages of gawk-4.0.2 `#D1709` 9771693
+- main: work around `XDG_RUNTIME_DIR` of a different user by `su` (reported by zim0369) `#D1712` 0000000
 
 ## Internal changes and fixes
 
diff --git a/note.txt b/note.txt
index 93728269..d0ae435f 100644
--- a/note.txt
+++ b/note.txt
@@ -5934,6 +5934,9 @@ bash_tips
     分からないので取り敢えずエラーメッセージを修正して何が表示されるか問い合わ
     せる事にする。
 
+    2021-12-19 分かった。'/run/user/1000/blesh' である。つまり、XDG_RUNTIME_DIR
+    が su する前のユーザーのディレクトリのままになっているのが問題である。
+
   * complete: FIGNORE で全てが棄却されるバグ (reported by seanfarley) [#D1711]
     https://github.com/akinomyoga/ble.sh/issues/162