To improve reproducibility of work accuracy under the same condition by predetecting an average release voltage value containing a variable impedance quantity, setting a correction value by its average release voltage value and a prescribed reference voltage value, and controlling relative moving speed of an electrode and a workpiece.
When an electric discharge machine is actuated, in an average voltage correcting part 20, first of all, in an inspection process, in a condition of keeping a discharge ungenerating distance between an electrode 1 and a workpiece 2, an interpole average voltage value measured by an average machining voltage detector 9, that is, an average release voltage value is taken in, and is stored in a storage part 21. In a machining process, a correction factor is decided on the basis of reference voltage and the average release voltage value stored in the storage part 21, and an average machining voltage value between the electrode 1 and the workpiece 2 is corrected by the correction factor, and is outputted to a machining speed control part 10. Here, a difference between the corrected average machining voltage value and the reference voltage is calculated, and a speed command value is decided so that this error voltage is made 0, and is outputted to a driving control device 11.