PURPOSE: To enable shortening of a machining time and to enable improvement of quality, by a method wherein a liquid leak preventing member is mounted to the peripheral edge of a surface plate or a frame, first and second machining tanks are formed and are switched by elevation of the tanks, and diemilling of a mold ranging from rough machining to finish is completed through one and the same device of one and the same electrode.

CONSTITUTION: When a frame body 8 is raised and a bottom member 6a of a machining tank 6 is brought into contact with a liquid leak preventing member 5 situated to the peripheral edge of the upper end of a frame 4, the bottom of the first machining tank 6 is formed with a surface plate 3, a liquid leak preventing member 5, and a bottom member 6a. When a switching valve 9 is switched, a solenoid valve 13 is opened and 14 is closed and a pump 12 is run, fresh electrolyte is fed from an electrolyte tank 11 to the tank 6. The pump 12 and the solenoid valve 13 are stopped once, and after data necessary to electrolytic treatment is inputted, energization and machining take place. When machining is progressed to a given level, energization and movement of an electrode are stopped, an electrolyte treating system pump 12, a solenoid valve 14, and an injection device 15 are stopped, and the valve 9 is switched to discharge waste liquid to a tank 10. The frame body 8 is then lowered and machined in a machining tank 7.