A tire air pressure detecting device has a tire air pressure detecting device on a wheel. A main transmitting device is provided at the wheel for transmitting an air pressure signal SP to a vehicle body. A main receiving device is provided at the vehicle body for receiving the air pressure signal SP. A main switch turn-on detecting device is provided on the vehicle body for detecting the turn-on of a main switch. A second control device is provided on the vehicle body for receiving the air pressure signal SP and a main switch ON signal Son. An auxiliary transmitting device on the vehicle body transmits a transmission instruction signal SR to the wheel when the second control means receives the main switch ON signal Son. An auxiliary receiving device on the wheel receives the transmission instruction signal SR. A first control device is provided for reading a new air pressure signal SP. When the main switch is turned on, the state of the tire air pressure can immediately be recognized. The main transmitting means is not required to transmit the air pressure signal at all times or increase the frequency with which to transmit the air pressure signal. Thus, the consumption of the battery at the wheel can be reduced.