JPH0575207B

PURPOSE:To immediately detect a fault in a transmission system including a signal transmitter and receiver by providing a device section for detecting vehicles under a decentralised condition and condensing detecting signals by means of the signal transmitter. CONSTITUTION:When an answering device 10 mounted on a vehicle, etc., approaches a questioning device 20 and is electromagnetically coupled with the questioning device 20, bits corresponding to the frequency components of frequencies f2 and f4, to which the answering device 10 resonates, and to the component of a frequency fM, to which an answering device 30 for marker resonates, become '1'. Contents of a shift register 50 including a marker bit are handed over to a transmitter 100 and the transmitter 100 transmits the contents to a receiver 110 by a prescribed transmitting method. Then the receiver 110 outputs a detecting signal to a central fault detector 120. The fault detector 120 discriminates that a fault occurs in the device from the marker bit which is not '1'.