PURPOSE: To enable unmanned operation, by providing an electrode surrounding wall on the top of a workpiece or a rest, supplying water into a machining tank while keeping the level of the water therein slightly lower than the top of the wall, and then machining the workpiece while supplying a liquid inside the wall, so as to prevent an accident.

CONSTITUTION: Throughout the machining of a workpiece 6, a machining liquid 22 is supplied from a machining liquid retrieval vessel 18 through a pump 16, a solenoid valve 14 and a machining liquid feeder 20. The machining liquid 22 contaminated with machined chips or denatured by electric discharge overflows a wall 8, floats on the top of water 21 contained in machining tank 4, and enters into an auxiliary tank 4a through the hole 5a of a partition plate 5. After the processing liquid is removed of the machined chips etc. by a filter 23, the liquid is retrieved into the machining liquid retrieval vessel 18. The quantity of the machining liquid required to be used for a machining section at the time of machining is thus minimized. Since the already used machining liquid is safely retrieved, an accident is prevented to enable unmanned machining for a long time.