A tire air pressure detection system and a vehicle body side device to detect air pressure of a plurality of tires. The vehicle body side device causes a transmission unit to transmit to all tire side devices, a transmission stop signal instructing transition to a transmission stop state and then the transmission unit transmits a transmission standby signal at a transmission strength that enables the signal to reach the tire side devices; and then the transmission unit transmits to the one of the tire side devices, a transmission request signal requesting a transmission of a signal including a unique identifier stored in that tire side device; and when it is determined that a reception unit has received a response signal in response to the transmission request signal, the transmission unit transmits a transmission standby signal and a transmission request signal to the tire side device corresponding to the next tire.