PURPOSE: To reduce the consumption of a working electrode and to improve the accuracy in working by providing a detection circuit detecting the suspension of an electric discharging during the electric discharging duration.

CONSTITUTION: On a down counter 12 the number of the pulses set on a set circuit 11 is set, the number of pulses impressed from an AND gate circuit 10 is subtracted and an arc cut time is measured. When a count up signal is impressed from a ring counter 9, the counting content so far is reset and the pulse number of the setting circuit 11 is set again. If the content of the down counter 12 becomes zero prior to the count up signal of the ring counter 9 being impressed, in other words, when the arc cut time within the time measured by the ring counter 9 becomes longer than the time set on the setting circuit 11, the count down signal is transmitted toward a monostable multivibrator 13 from the down counter 12, a light emitting diode 15 is lit to display the abnormality of the working state.