PURPOSE: To facilitate to detect the condition of gap and improve working performance by providing a switching element between a capacitor, accumulating discharging energy, and the working gap.

CONSTITUTION: When a work starting switch 21 is put ON, charging of the electric discharging capacitor 23 is begun and the charging voltage E1 is compared with a reference voltage E0 by a comparating circuit 38, and when the charging is finished, the switching element 31 is put ON. When the electric discharge is generated in the working gap and the detecting voltage E1 becomes smaller than the reference voltage E3', the switching element 31 is put OFF by the output of the comparating circuit 43. A timer 42 begins counting upon starting of the electric discharge in order to prohibit next ON signal of the switching element until a predetermined time has elapsed. The capacitor is connected after the switching element, therefore, the working gap may be independent, the detection thereof may be facilitated and the working performance may be improved.