A wheel position detecting device and a tire air pressure detecting device including the wheel position detecting device are provided. The wheel position detecting device includes triggering devices and a receiver mounted on the vehicle-body side, and transmitters mounted on the wheels. Each triggering device outputs a trigger signal to each transmitter. In response to the trigger signal, each transmitter transmits a frame to the receiver. The receiver receives the frame and judges whether the transmitter is mounted on a subject wheel, based on whether reception strength indicated by reception strength data stored in the frame is within a range set in advance.