PURPOSE: To stabilize control signal deciding operation by using the counted time of a counter for deciding a control signal.

CONSTITUTION: The control signal passed through a band-pass filter 11 is compared with the reference voltage of a reference voltage source 13 and sent out as a rectangular wave signal. This rectangular wave signal is sent out to the set terminal of a flip-flop 14 and also to the reset terminal of the counter 15 to prevent the flip-flop 14 from being reset. The counter 15 starts counting for a specific time once the level at its reset terminal varies from H to L to send out an H signal to its output terminal when the level at the reset terminal does not go down to H again. Consequently, the output of the flip-flop 14 is held at H as far as the rectangular wave of the control signal is inputted within the specific time, and a counter 16 continues to count the holding time.