To prevent unnecessary noise and reduce the power consumption by stopping the operation of a binarizing means when the AM component included in a received radio wave is small in a point separated from a transmitting post, and fixing the output to prevent the unstable operation of the binarizing means.

In a point separated from a transmitting post, the receiving level is lower than a standard level, a level detecting means 1 outputs no AM control signal, and a binarizing means is thus not operated. When a receiving vehicle approaches the transmitting post, and the receiving level is raised to the standard level or more, the means 1 outputs an AM control signal. The binarizing means 3 is operated according to the AM control signal. When the AM component exceeds the AM demodulation threshold level, the demodulation waveform of the means 3 is changed to a binarized waveform according to the AM component included in the received radio wave, and when the AM component is lower than the AM demodulation threshold level just under the transmitting post, the demodulation waveform of the means 3 is disturbed. The means 3 is operated until the receiving level is reduced below the standard level through the transmitting post, and the AM control signal of the means 1 is eliminated.