diff --git a/LICENSE b/LICENSE
index 272e67a..933b6af 100644
--- a/LICENSE
+++ b/LICENSE
@@ -1,6 +1,6 @@
 MIT License
 
-Copyright (c) 2025 beatinaniwa
+Copyright (c) 2026 beatinaniwa
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal
diff --git a/README.md b/README.md
index 86764a8..f01c1f9 100644
--- a/README.md
+++ b/README.md
@@ -189,15 +189,14 @@ mf journals create --json '{"..."}' --dry-run
 
 ### 設定ファイル
 
-設定ファイルの保存場所は OS の `UserConfigDir` に基づきます:
-
-| OS | パス |
-|----|------|
-| macOS | `~/Library/Application Support/mf-cli/` |
-| Linux | `~/.config/mf-cli/` |
-| Windows | `%AppData%\mf-cli\` |
-
-`MF_CONFIG_DIR` 環境変数でオーバーライドできます。
+設定ファイルの保存場所は [XDG Base Directory](https://specifications.freedesktop.org/basedir-spec/latest/) に準拠します:
+
+| 条件 | パス |
+|------|------|
+| `MF_CONFIG_DIR` 設定時 | `$MF_CONFIG_DIR/` |
+| `XDG_CONFIG_HOME` 設定時 | `$XDG_CONFIG_HOME/mf-cli/` |
+| デフォルト（macOS/Linux） | `~/.config/mf-cli/` |
+| デフォルト（Windows） | `%AppData%\mf-cli\` |
 
 #### config.json
 
diff --git a/internal/config/config.go b/internal/config/config.go
index bade548..88fab25 100644
--- a/internal/config/config.go
+++ b/internal/config/config.go
@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"runtime"
 	"strconv"
 )
 
@@ -73,9 +74,19 @@ func resolveConfigDir() (string, error) {
 	if v := os.Getenv("MF_CONFIG_DIR"); v != "" {
 		return v, nil
 	}
-	base, err := os.UserConfigDir()
+	if runtime.GOOS == "windows" {
+		base, err := os.UserConfigDir()
+		if err != nil {
+			return "", err
+		}
+		return filepath.Join(base, "mf-cli"), nil
+	}
+	if v := os.Getenv("XDG_CONFIG_HOME"); v != "" && filepath.IsAbs(v) {
+		return filepath.Join(v, "mf-cli"), nil
+	}
+	home, err := os.UserHomeDir()
 	if err != nil {
 		return "", err
 	}
-	return filepath.Join(base, "mf-cli"), nil
+	return filepath.Join(home, ".config", "mf-cli"), nil
 }
diff --git a/internal/config/config_test.go b/internal/config/config_test.go
index deeed95..cd4d366 100644
--- a/internal/config/config_test.go
+++ b/internal/config/config_test.go
@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"os"
 	"path/filepath"
+	"runtime"
 	"testing"
 )
 
@@ -141,6 +142,63 @@ func TestLoad_InvalidJSON(t *testing.T) {
 	}
 }
 
+func TestResolveConfigDir_XDGConfigHome(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("Windows does not recognise /custom/xdg as absolute")
+	}
+	t.Setenv("MF_CONFIG_DIR", "")
+	t.Setenv("XDG_CONFIG_HOME", "/custom/xdg")
+
+	dir, err := resolveConfigDir()
+	if err != nil {
+		t.Fatalf("resolveConfigDir() returned error: %v", err)
+	}
+	want := filepath.Join("/custom/xdg", "mf-cli")
+	if dir != want {
+		t.Errorf("resolveConfigDir() = %q, want %q", dir, want)
+	}
+}
+
+func TestResolveConfigDir_DefaultFallback(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("default fallback on Windows uses AppData, not ~/.config")
+	}
+	tmpHome := t.TempDir()
+	t.Setenv("HOME", tmpHome)
+	t.Setenv("MF_CONFIG_DIR", "")
+	t.Setenv("XDG_CONFIG_HOME", "")
+
+	dir, err := resolveConfigDir()
+	if err != nil {
+		t.Fatalf("resolveConfigDir() returned error: %v", err)
+	}
+
+	want := filepath.Join(tmpHome, ".config", "mf-cli")
+	if dir != want {
+		t.Errorf("resolveConfigDir() = %q, want %q", dir, want)
+	}
+}
+
+func TestResolveConfigDir_RelativeXDGIgnored(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("XDG fallback test not applicable on Windows")
+	}
+	tmpHome := t.TempDir()
+	t.Setenv("HOME", tmpHome)
+	t.Setenv("MF_CONFIG_DIR", "")
+	t.Setenv("XDG_CONFIG_HOME", "relative/path")
+
+	dir, err := resolveConfigDir()
+	if err != nil {
+		t.Fatalf("resolveConfigDir() returned error: %v", err)
+	}
+
+	want := filepath.Join(tmpHome, ".config", "mf-cli")
+	if dir != want {
+		t.Errorf("resolveConfigDir() = %q, want %q (relative XDG_CONFIG_HOME should be ignored)", dir, want)
+	}
+}
+
 func TestRequireClientID_Empty(t *testing.T) {
 	cfg := &Config{ClientID: ""}
 	if err := cfg.RequireClientID(); err == nil {