A main control portion of a vehicle has an LF transmission antenna for transmitting a search signal. The LF transmission antenna has five antennas respectively provided on door knobs of the vehicle. A smart key receives the search signals transmitted from respective antennas constituting the LF transmission antenna, and then measures a filed strength of the received search signal. The smart key transmits field strength information representing the measured field strength to the main control portion. The main control portion calculates relative position information of the vehicle to the smart key, based on the field strength information. On the basis of the relative position information, the main control portion controls the door of the vehicle to be locked or unlocked.