PURPOSE: To remove the effect of the variation of the amplitude value of an input signal by comparing reference voltage proportional to the peak voltage of an AC input and an AC input signal and generating a phase detecting signal having a predetermined phase difference to the AC input signal.

CONSTITUTION: An AC input signal applied to an input terminal 1 is full-wave rectified by a full-wave rectifying circuit 2, and transmitted over a comparator 5 while being sent to a peak-voltage holding circuit 3. Reference voltage proportional to the peak voltage of the full-wave rectified AC input signal is generated by the peak-voltage holding circuit 3 and a voltage-dividing circuit 4, and the reference voltage is transmitted over the comparator 5. An oscillating circuit 6 is driven by the variation such as rise of output voltage from the comparator 5, and two-phase pulses are outputted as a phase detecting signal from the oscillating circuit 6.