PURPOSE: To prevent the effect of an interference wave on an FSK demodulation circuit, by providing a comparator comparing an output of a BPF corresponding to each frequency with a threshold value, and interrupting a demodulation comparison circuit when the said level exceeds the threshold value.

CONSTITUTION: An FSK modulation signal obtained from a reception circuit 15 passes through two BPFs 3, 4 and inputted to a comparison circuit 11 via envelope detection circuits 7, 8. The comparison circuit 11 forms a binary signal corresponding to a detection output and gives it to an instruction decoder circuit 13. Other envlope detection lines 16, 17 are connected to the output of the BPFs 3, 4, and the detected outputs are compared with a threshold value at comparators 20, 21. When either one of the comparators has an output, any of the comparators 7, 8 corresponded is grounded with transistors 26, 27 and the comparison circuit 11 takes this ground level as a comparison circuit and democulates an FSK signal. Thus, the demodulation not affected with interference signals can be performed.