A vehicle-side device receives a response signal transmitted from an electronic key carried by a user and executes a matching process to check whether the key is an authorized one. When the matching is confirmed and the operation of a sensor/switch disposed on a door is detected, a given door is unlocked. The device determines a door which is to be opened by the user, and the seat state is controlled on the basis the door to be opened. If the door to be opened is for rearmost seats, the rearmost seats are automatically controlled to a seating state or a loading state based on detection of absence or presence of an object on the rearmost seats.