diff --git a/routes/ponshu.js b/routes/ponshu.js
index b83a2ed3..33bf9932 100644
--- a/routes/ponshu.js
+++ b/routes/ponshu.js
@@ -40,12 +40,28 @@ module.exports = function createPonshuRouter(pool, authMiddleware) {
                 });
             }
 
-            // 初回アクティベート
+            // 初回アクティベート — WHERE device_id IS NULL で競合を防ぐ
             if (!license.device_id) {
-                await pool.query(
-                    `UPDATE ponshu_licenses SET device_id = $1, activated_at = NOW() WHERE license_key = $2`,
+                const activateResult = await pool.query(
+                    `UPDATE ponshu_licenses SET device_id = $1, activated_at = NOW()
+                     WHERE license_key = $2 AND device_id IS NULL
+                     RETURNING license_key`,
                     [device_id, license_key]
                 );
+                if (activateResult.rowCount === 0) {
+                    // 別リクエストが先にアクティベートした → 再取得して照合
+                    const retry = await pool.query(
+                        `SELECT device_id FROM ponshu_licenses WHERE license_key = $1`, [license_key]
+                    );
+                    const current = retry.rows[0];
+                    if (!current || current.device_id !== device_id) {
+                        return res.json({
+                            valid: false,
+                            error: '別のデバイスで登録済みです。端末変更の場合はサポートまでご連絡ください。',
+                            supportEmail: APP_SUPPORT_EMAIL,
+                        });
+                    }
+                }
                 console.log(`[Ponshu] License activated: ${license_key.substring(0, 12)}... -> Device: ${device_id.substring(0, 8)}...`);
                 return res.json({ valid: true, plan: license.plan, activated: true });
             }