PURPOSE: To obtain a sufficient detection output with a simple LPF by eliminating an undesired AC component included in a pulse signal generated from a pulse generating circuit while using a subtractor circuit.

CONSTITUTION: An FM signal inputted to an FM signal input terminal 1 is shaped into a rectangular wave by a limiter amplifier 2. A monostable multivibrator 3 generates a pulse signal synchronously with the leading of a rectangular wave outputted from the amplifier 2. A subtractor circuit 6 subtracts an inverted input signal from a non-inverting input signal. In using an LPF 4 to smooth the output of the circuit 6, a DC voltage proportional to the input signal frequency is obtained at a detection output terminal 5 as a detection output. Through the constitution above, the AC component included in the output signal of the monostable multivibrator 3 is eliminated by the circuit 6. The constitution of the circuit 4 is very simple and the increase in number of components is not so many even with the addition of the circuit 4. While the effect of reduction in the number of component is large by simplify the LPF 4, the circuit is suited to circuit integration.