PURPOSE: To obtain a demodulation circuit not effected with the duty of a modulation waveform, by inputting a frequency modulation wave to two monostable multivibrator circuits with different output pulse width and resetting one monostable multivibrator circuit with the other circuit output.

CONSTITUTION: A frequency modulation wave (a) is inputted to monostable multivibrator circuits 1, 2 having output pulse widths of TW1 and TW2. The output pulse widths TW1, TW2 have the relation with "0", "1" frequencies f1, f2 as 1/f2<1/f1, and TW2>1/f2, 1/f1, and an output of the monostable multivibrator circuit 1 has a signals having notches corresponding to the frequencies f1, f2 and continuous signal (b). The output of the circuit 1 is connected to a reset terminal of the circuit 2 and with the frequency f1, reset is given and a demodulation signal (c) appears at the output of the circuit 2.