PURPOSE: To stably and accurately demodulate an FSK signal even when the FSK signal with consecutive '0' or '1' is inputted by sampling and holding part of a voltage signal obtained by demodulating a basic frequency signal so as to use the result as a reference value of a comparator or a subtractor.

CONSTITUTION: Outputs of the 1st data selector 1 and the 2nd data selector 2 are demodulated respectively by the 1st and 2nd FM demodulators 3, 4 and voltage signals (g),(h) having a voltage level in response to the frequency are outputted. The voltage signal (g) is fed to the 1st sample holding circuit 5, where the signal is sampled in the timing of a sampling signal (c) and the intermediate voltage of the part of the fundamental frequency (fo) in the voltage signal (g) is inputted to the 1st comparator 7 as a reference value. Since part of the voltage signal obtained by demodulating the intermediate frequency signal between the mark and the base is subjected to sample-and-hold to form a reference value of the comparator in this way, the demodulator is stably operated against the temperature change and even when space signals or mark signals are inputted consecutively, accurate demodulation is applied.