PURPOSE: To prevent damage of an electrode, by detecting the degree of insulation of an insulative machining fluid existing in the interpole gap and by comparing thus detected value with a reference value to control servo-reference voltage during the length of the gap between the electrode and a workpiece is servocontrolled.

CONSTITUTION: High frequency voltage is superposed during a deionizing time or the quiescent time of pulse voltage applied across the interpole gap from a machining power source to detect interpole leakage current running due to the lowering of insulation of the machining fluid 3. When thus detected current value exceeds a predetermined value, a control instruction signal generating device delivers a danger signal SA to a control means 30. Further, when the signal SA is high, which means that an abnormal condition occurs in the interpole gap, switches 101, 102 are turned on and off, respectively, by means of an inverter 100, and reference voltage Vr is continuously increased by means of an integrating circuit composed of an operation amplifier 103 and the like. Accordingly, the averaged voltage V across the interpole gap is increased in the negative direction, and therefore, the interpole gap is enlarged due to a variation in the output of an output circuit 106. Thus, it is possible to aim at enhancing the rate of machining and to surely prevent damage of the electrode.