PURPOSE: To enable even high resistance workpiece to be quickly and efficiently subjected to electric discharge machining by applying high voltage between an electrode and the workpiece to supply large discharge current and shutting off the high voltage application automatically a short time after starting the discharge.

CONSTITUTION: A high resistance workpiece 3 of conductive ceramics or the like is disposed in a working liquid 1 and for example 500W1000V of high voltage pulses having short pulse width are supplied between the workpiece and a wire electrode 2 from a high voltage switching power source 10 and the wire electrode 2 is moved in the direction a for performing electric discharging machining. Then, 1W5μS after rising time due to the start of discharge, the high voltage pulses forcubly shut off the high voltage application to supply large discharge current for a short time and provide 2000W50000times/sec. of discharge number of times. Thus, even high resistance workpiece can be worked rapidly and efficiently by electric discharge machining.