A tire air pressure monitor device is provided for monitoring air pressure in tires using pressure sensors, which are installed in the tire of each wheel. Each wheel has a transmitter for wirelessly transmitting the air pressure information along with a sensor ID when the wheel is at a predetermined rotation position. A vehicle body has a receiver for receiving the wireless signals. A wheel speed sensor is provided on a vehicle body for each wheel to determine the wheel rotation position. A TPMS control unit acquires the wheels rotation position when a wireless signal containing a certain sensor ID is transmitted. The TPMS control unit accumulates wheel rotation position data, and determines the wheel position corresponding to the wheel rotation position data having the smallest degree of dispersion from among the wheel rotation position data as the wheel position for the transmitter corresponding to the sensor ID.