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 judges whether reception strength data is correct or incorrect based on whether reception strength indicated by the reception strength data (the reception strength data expresses the reception strength of a trigger signal transmitted from a receiver, provided in a vehicle body, to a receiver of a transceiver on each wheel) transmitted back to the vehicle-mounted receiver from each wheel-mounted transceiver is within a range set in advance. A transceiver is provided on each wheel. The wheel position detecting device judges whether to decide the wheel position using the reception data.