A tire air pressure detecting apparatus has an tire air pressure detecting mechanism for detecting the air pressure of a tire of a motorcycle, and a second control mechanism for comparing an actual air pressure detected by the tire air pressure detecting mechanism with a stored air pressure threshold, and controlling an engine output when the actual air pressure becomes lower than said air pressure threshold. Since the air pressure detecting apparatus comprises a small number of constituent elements including the tire air pressure detecting mechanism and the second control mechanism for reducing an engine output under certain conditions, the tire air pressure detecting apparatus can easily be reduced in size and cost.