PURPOSE: To increase the demodulation sensitivity by adding a circuit bringing a pulse width τcof an output wavefo/n of a pulse count circuit into a pulse width τc-at (At is a pulse interval).

CONSTITUTION: The 1st pulse generating circuit 11 comprising a monostable multivibrator generates a pulse having a constant pulse width τc at the leading of an input signal. The output of the circuit 11 is a pulse train of a prescribed pulse width τC and its pulse interval t is a function of an input frequency fin. Th pulse width of the input pulse τc is narrowered by a time proportional to the pulse interval t in the 2nd pulse generating circuit 12 to generate a pulse train having a width of τc-at. In integrating the pulse train through an LPF 13, the output is obtained as V0=E[(a+1)τcfin-a]. The demodulation band of the lower frequency side is narrowerd in the characteristic as shown in figure and the demodulation sensitivity is increased by a multiple of (a+1) by the narrowered value. Thus, when an input signal with a comparatively narrower band is used, the titled system is advantageous.