PURPOSE: To prevent the consumption of an electrode and to make it possible to carry out an accurate corner machining, by charging a gas or liquid containing hydrocarbon or a substance which generates the same gas or liquid upon heating, in a closed-cave formed in a porous material electrode.

CONSTITUTION: When a machining process is carried out with the repetition of electric discharge under application of a pulse voltage across an opposed gap from a machining power source, a gas or liquid containing hydrocarbon charged in a machining cave 2 in an electrode 1 which is set to be opposed to a workpiece is oozed out to the outer surface of the electrode 1 through pores in the electrode 1, and is stuck thereon. Due to the ooze-out of hydrocarbon, the latter is always present on the outer surface of the electrode, and is exposed to machining electric discharge by which hydrocarbon is decomposed and ionized so that it is deposited on the electric discharge electrode to form a graphite deposited layer. Particularly, this graphite is liable to be deposited by a large amount to a corner part to which the graphite is easily concentrated during electric discharge, in proportion to the generation of electric discharge. Thus, the consumption of surface of the electrode may be prevented, which is particular to the corner part where is liable to be easily consumed and deformed, thereby an accurate electric discharge machining process may be made.