PURPOSE: To make the demodulation amplitude level constant without requiring high stability for a monostable multivibrator by adding a pulse level control circuit to an FM modulation circuit using zero cross detection.

CONSTITUTION: The DC component 13 of a demodulation LPF2 is detected by a DC component extracting circuit 10. An error amplification DC amplifier 11 amplifies a difference between the DC component 13 and a reference voltage 15 to generate an AGC voltage 14. The voltage 14 controls a variable gain amplifier 9 to control the level of the pulse inputted to the LPF2. Even if the width and level of the output pulse of the monostable multivibrator 1 are fluctuated, the width and level are made constant by the AGC circuit. Thus, the demodulated output 5 obtained from the LPF2 is made constant. The demodulation amplitude level is made constant without requring high stability for the monostable multivibrator.