PURPOSE: To prevent generation of abnormal arc discharge by detecting the shock wave generated when any electric discharge takes place in a working gap and discriminating the working state according to the magnitude of said shock wave, thus controlling the movement of an electrode.

CONSTITUTION: When a workpiece 14 is subjected to electric discharge work by an electrode 10, shock waves are generated at the moment when electric discharge is generated in a working gap 20 and are propagated. The magnitude of said shock wave is large during normal electric discharge and is small during abnormal arc discharge. The detection signal for the shock wave detected by a sensor 35 and the detection signal for the discharge current detected by a current detector CT are inputted into a discriminating apparatus 40, and normal discharge and abnormal arc discharge etc. are discriminated. When abnormal arc discharge takes place successively a prescribed number of times or more, an electrode driving apparatus 26 is controlled to carry-out the elevation movement of electrode, and thus the time, speed, magnitude etc. of the elevation movement are automatically set in accordance with the degree of anomaly.