PURPOSE: To obtain a frequency discriminating output having excellent linearity from an average DC voltage of a pulse signal difference, by shaping an input pulse signal and detecting the difference of the pulse width between the input signal and a pulse signal generated in synchronizing with the input signal.

CONSTITUTION: An input signal ei is applied to a pulse shaping circuit 22 via an input terminal 21 and the shaped signal is applied to a positive/negative comparison circuit 29. A pulse generating circuit 33 generates a reference pulse signal Pc in synchronizing with the shaped pulse signal Ps. NAND circuits 30, 32 compare the shaped pulse signal Ps with the pulse width of the Pc, supply output signals PP, PM to integration circuits 50, 51 via inverter circuits 42, 43, and the signals are converted into a DC voltage.