PURPOSE: To make it possible to discriminate the interpole condition without lowering the rate of machining, by superposing high frequency alternate voltage during the quiescent time of pulse voltage applied to the interpole gap so that the degree of insulation of insulative machining fluid is detected in accordance with the leakage current across the interpole gap.

CONSTITUTION: A control instruction signal generating device 17 superposes high frequency alternate voltage from an electrical sources 18 during a deionizing period or the quiescent time of pulse voltage applied between a wire electrode 2 and a workpiece 1 from a machining power source 15, and a current detector 6 detects interpole leakage current running due to the lowering of insulation of the machining fluid 3 which is caused by machined chips. Thus detected current value is delivered to a detecting circuit 119 through an amplifier 117 and a switch 118 which is turned on during the quiescent time. An output power SE obtained through envelope detection is compared with preset reference values V1, V2 by comparators 120, 121, and the degree of insulation is determined by means of an AND gate 22 to perform predetermined control. Thus, it is possible to appropriately discriminate whether the condition of machining is satisfactory or not without lowering the rate of machining, thereby it is possible to prevent accidents due to breakage of the wire electrode.