Each tire pressure detector detects a characteristic point in a detection signal of an acceleration sensor indicating that the tire pressure detector is located at a certain rotation position. When detecting the characteristic point, the tire pressure detector transmits a notification signal including the detector ID. Whenever a receiver receives a notification signal, the receiver acquires an axle rotation amount of a representative wheel and stores the axle rotation amount in a memory in association with the detector ID in the notification signal. After acquiring axle rotation amounts associated with the detector IDs of all of the tire pressure detectors, the receiver determines mounting positions of tires from the angle each tire pressure detector rotated and the angle each axle rotated between a first determination timing and a second determination timing.