To provide a system which dispenses with conscious labor of a driver and makes battery consumption minimum.
This is the device and method alarming to the driver that the door is locked and the key is left in the vehicle. A smart-key 10 is provided with a built-in transceiver 12, a battery 11, and a mechanical switch 14 making the transceiver not act for saving key battery power. This device and method cover four conditions after closing a vehicle door and being locked. The first condition is that driver is in the vehicle. The second is that the driver is outside the vehicle and has the key. The third is that the driver leaves the key in an igniton 16. The fourth is that the driver leaves the key on somewhere except the ignition. The third and fourth conditions must be alarmed to the driver. This system operates an alarm 24 to inform the driver, and temporarily unlocks the door.