To prevent interference of a response signal in a tire state monitoring system in which a detector on the wheel side transmits the response signal according to a request signal from the monitoring device side.

In a tire monitoring system in which a monitoring device on the body side transmits the request signal from transmitters near each wheel, detectors provided to each wheel transmit response signals including detection results of tire air pressure and temperature of the wheels when the request signal is received and the monitoring device monitors tire states of each wheel based on the response signals received by a receiver on the body side, each detector is constituted so as to transmit the response signals (S230) when it is judged that a threshold is smaller than an RSSI value indicating a value of field intensity of the request signal received by the detector (S210: YES). Then, the threshold is set to a value in which the RSSI value becomes larger than the threshold only when the detector receive the request signal from the transmitter for wheels corresponding to itself.