PURPOSE: To increase the treatment speed and the maximum treatment speed for a rough surface, by lowering power source voltage, and at the same time increasing an electric current peak value and narrowing a pulse width.

CONSTITUTION: A signal is issued from a timing control circuit 24, and when an output voltage E1 of a subsidiary power source V1 is given to an electrode 16, delayed discharge occurs at every pulse under the influence of the inductance, etc. of a circuit l. The occurrence of discharge is detected by a comparing means in the timing control circuit 24, and a signal is outputted. By this signal a main switching circuit S2 is closed only to make it correspond to its narrow pulse width, only when a high voltage E2 of the main power source V2 having a great peak electric current capacity is energized to the electrode 16 to increase the treatment speed.