An apparatus for detecting positions of wheels of a vehicle is provided. In the apparatus, a triggering signal is transmitted from the body side to each transceiver mounted at each wheel. In each transceiver, the strength of the received triggering signal is measured and formatted into a frame of data to be transmitted to a receiver mounted to the vehicle body. The strength of the triggering signal is used to detect wheel positions by determining that each transceiver is disposed at which wheel. The receiver determines if there is a reply of the frame of data containing the strength when the triggering device transmits the triggering signal. When no reply comes from the transceiver, the strength of noise is measured in the transceiver in response to a command from the receiver. When the noise strength is less than a threshold, the receiver commands an alarm member to issue an alarm.