To improve transmission reliability by realizing an optimum frequency selection function with a simple configuration in the one-way communication system that selects an optimum communication frequency.

A reception antenna 7 of the receiver receives a frequency- modulated signal, a phase locked loop circuit 29 demodulates the received signal and outputs the demodulated signal to a central processing unit 28. The phase locked loop circuit 29 usually locks the frequency of reception to the frequency of the received signal from a phase locked loop control circuit 25. However, when the phase locked loop control circuit 25 detects a demodulation signal (demodulation signal of a non-modulation carrier signal) with a prescribed level or over, the phase locked loop control circuit 25 fixes the reception frequency of the phase locked loop circuit 29.