The tire air pressure sensor is an integrated structure with the air valve and installed to each tire. Each tire air pressure sensor has its own ID and the IDs for the 4 tires are registered in the wireless access module. The tire air pressure sensor sends the sensor ID, tire air pressure, tire temperature, wheel rotation status data, and sensor status data to the wireless access module through radio waves. The wireless access module monitors a voltage decrease in the battery based on the sensor status data received from the tire air pressure sensor. If the wireless access module receives a voltage decrease data in the battery from the right front tire air pressure sensor, the DTC is set.