PURPOSE: To prevent degradation of the nature of a worked surface due to an abnormal discharge by applying a discharge voltage lower than a machining voltage for cleaning an interpole space to forcedly cause the abnormal discharge caused by chips or residual ion between both electrodes while the machining voltage is applied to a work.

CONSTITUTION: At intervals between the applications of a discharge voltage for machining a work by a first machining power supply 18, a discharge voltage for cleaning an interpole space is applied to the work to cause a secondary discharge due to chips or residual ion between both electrodes by means of a machining power supply 19. By the secondary discharge, the chips or residual ion can be removed, and the generation of an abnormal discharge adversely affecting the electric discharge machining can be also restricted, with the result that the work W can be electro-discharge machined with high accuracy. When the secondary discharge has not been generated, the discharge pulse oscillations of the discharge voltage are increased, and the discharge machining of the work W by a wire 2 is performed at high speeds.