Tire air pressure detection system and vehicle body side device

A tire air pressure detection system comprising:

• a tire side device, provided in each of a plurality of tires mounted on the vehicle, each tire side device including a sensor that detects the air pressure in the corresponding tire, a tire side reception unit that wirelessly receives a signal requesting the sensor to transmit a measurement result, and a tire side transmission unit that wirelessly transmits the measurement result in response to the request;
• and
• a vehicle body side device, provided in a body of the vehicle, the vehicle body side device including a vehicle body side transmission unit and a vehicle body side reception unit that wirelessly transmit and receive signals to and from the tire side devices, and
• the vehicle body side device acquiring the air pressure in each tire and detecting a decrease in the air pressure,
• wherein each tire side device includes:
• a storage unit that stores an identifier identifying that tire side device;
• and
• a control unit that stops the transmission by the tire side transmission unit in the case where a transmission stop signal instructing a transition to a transmission stop state has been received by the tire side reception unit, and that, after the tire side reception unit has received a transmission standby signal instructing a transition to a transmission standby state, causes a response signal including the identifier to be transmitted from the tire side transmission unit in response to the transmission request signal,
• the vehicle body side device includes:
• a transmission control unit that causes the transmission stop signal to be transmitted from the vehicle body side transmission unit to all of the tire side devices of the plurality of tires, then transmits the transmission standby signal from the vehicle body side transmission unit at a transmission strength that reaches the tire side device in one of the plurality of tires, and then causes the transmission request signal to be transmitted from the vehicle body side transmission unit;
• and
• a determination unit that determines whether or not the vehicle body side reception unit has received a response signal in response to the transmission request signal, and
• in the case where the determination unit determines that the response signal has been received, the transmission control unit causes the vehicle body side transmission unit to transmit the transmission standby signal at a transmission strength that reaches the tire side device of the next tire, and then to transmit the transmission request signal, to the tire side device.