To accurately determine whether a portable unit is in a cabin or taken out of the cabin.

When communication with a portable unit is performed by cabin interior antennas 1, 2, the portable unit is determined to be in the cabin, and when communication with the portable unit is not performed by the cabin interior antennas 1, 2 but performed by cabin exterior antennas 3-6, the portable unit is determined to be taken out of the cabin. When communication with the portable unit is performed by neither the cabin interior antennas 1, 2 nor the cabin exterior antennas 3-6, the portable unit is determined to be in the cabin.