Four transceivers are attached to respective wheels of a vehicle. A pair of triggering devices transmit respectively different types of trigger signal, respectively directed toward the front wheels and toward the rear wheels. One triggering device is located closer to one rear wheel than to the other, while the other triggering device is located closer to one front wheel than to the other. Based on the received trigger signal type, a transceiver judges itself to be attached to either a front or a rear wheel, and based on the received trigger signal strength, judges whether it is attached to a right-side or left-side wheel.