A vehicle has a reference receiver installed at an outer edge position of a desired detection area in a compartment to receive a signal transmitted from a transmitter in the compartment. The level of signal transmission output of the transmitter is adjusted based on the reception result of the reference receiver. It becomes possible to adjust the signal transmission output of the transmitter in the compartment so as to obtain a desired detection area at all times irrespective of change of the detection area reached by the request signal from the transmitter determined by the compartment or irrespective of variation in the electromagnetic wave environment in which the vehicle is placed.