PURPOSE: To shorten the time spent in detection of fall of the signal level by detecting the fall of an AC input signal to a level lower than a prescribed level in accordance with the change of a smoothed signal.

CONSTITUTION: A signal (a) inputted to an input terminal 1 is subjected to full- wave rectification, and a high-level signal is outputted when the fall to a level lower than a prescribed level is detected by a level comparator 11. A pulse generator 12 outputs a high-level signal for only a certain period T when the signal of the level comparator 11 is switched from the low level to the high level, and the generator 12 outputs a low-level signal thereafter. This certain period T is set to a time longer than a half period of the input signal, and intervals of continuous pulses, which are generated when the input signal has a level higher than the certain level, are reduced to zero, and a full-wave rectified waveform (b) is converted equivalently to a smoothed waveform (c), and the fall of the input signal to a level lower than the prescribed level is detected by the change of this smoothed waveform (c).