PURPOSE: To detect power failure in a short time so as to prevent any wasteful electric discharge of a condenser by providing a power failure discriminating circuit for receiving the primary input terminal voltage of a rectifier circuit so as to detect whether commercial frequency is included in the inputted voltage every cycle of the commercial frequency.

CONSTITUTION: An automatic water discharge device is provided with a power failure discriminating circuit 11 for discriminating the existence of power failure, and this power failure discriminating circuit 11 is formed of a comparator 12, a pulse detecting circuit 13 and a clock circuit 14. The comparator 12 receives terminal voltage Va from one of the primary input terminals of a rectifier circuit 2, and compares it with the reference voltage Vb, and the output of the comparator 12 is turned on only during Va > Vb. A pulse signal of commercial frequency thus comes into existence. The pulse detecting circuit 13 counts high speed clock signals from a clock circuit 14, and judges power failure in case the 'on' state is not generated to the output of the comparator 12 during one cycle of the commercial frequency. This judgement of power failure is outputted to a power failure time valve closing circuit 15 to close a solenoid valve 4.