PURPOSE: To make it possible to control to disperse localized discharge by arranging a potential detection means between two conductor pieces and further determining that localized discharge is occurring when a potential change detection means detects no potential change.

CONSTITUTION: When a potential V1-V2 between upper and lower conductor pieces 3, 4 relative to a work 2 is inputted to a differential amplifier 15 and the start of discharge is detected with a comparator 16, a discharge start time potential detection means consisting of a sampling hold circuit 17, and a smoothing circuit 18 detects a potential at the time a discharge current is built up at the time of the discharge start. Detecting whether or not the detected potential is changing by means of a potential change detection means consisting of comparators 19, 20 and timers T2WT5, it is determined to be a normal discharge when there is a change in the potential, and to be a localized discharge when there is no change in the potential. When the localized discharge is detected in this way, machining conditions such as on-off time of the machining power source, etc., are changed to disperse the localized discharge.