PURPOSE: To detect the frequency difference of an intermittent wave with high accuracy, by generating a sawtooth wave in response to the phase difference between a reference signal and an input signal, comparing the sawtooth wave with a different threshold value for generating plural pulse signals, and generating a discriminating signal thereby.

CONSTITUTION: An oscillating output signal from an oscillator 3A is applied to a reset terminal R of a flip-flop circuit 11 and a reference output signal from an oscillator 12 is applied to a set terminal S. An output of the flip-flop circuit 11 is inputted to a low pass filer 16 via a subtractor 13 and a switch circuit 14 to obtain the sawtooth wave signal, which is supplied to Schmitt circuits 17, 18 having two different threshold values, and after a prescribed pulse signal is formed, the signal is supplied to a flip-flop circuit 19 as a discriminating signal generating means. After the output is converted into a DC voltage at a diode 20 and a capacitor 22, the result is applied to the oscillator 3A as a control voltage.