PURPOSE: To control a discharge current easily and to shorten the time from an oscillation stop to detection by using a depletion type N-MOS and discharging its charges as a drain-source current.

CONSTITUTION: While a terminal C is at "H", the depletion type N-MOS9 turns on sufficiently and the output of an inverter 2 at this time is at "L", so a terminal D is at "L", charging a capacitor 6 to "L". When the terminal C is at "L", on the other hand, the gate of the N-MOS9 is held at "L". Charges in the capacitor 6 flow to the N-MOS9 as the drain-source current. Consequently, the discharge current is controlled by the channel length, channel width, and threshold value of the transistor, and the time constant of discharge is designed easily, shortening the time from an oscillation stop to detection.