PURPOSE: To accurately make numerous grooves simultaneously on a workpiece, by tightly fitting an electrode with a cylindrical electric insulator sleeve having slits corresponding to the grooves, by tightly fitting the workpiece on the sleeve, and by supplying a liquid electrolyte to the slits through the passages of the electrode to electrically machine the workpiece.

CONSTITUTION: Electricity is applied as an electrode 6 and a cylinder 8 are used as cathode and anode, respectively. A liquid electrolyte is introduced through the central through hole 74 of an electric insulator rod 7 so that the electrolyte flows through the internal opening 64 and radial passages 65 of the electrode 6 and enters into the slits 91 of an electric insulator sleeve 9. As a result, electrical currents flow through the electrolyte in the slits 91 between the electrode 6 and the cylinder 8 to electrolytically make grooves on the inside surface of the cylinder 8. Since the form of each slit 91 of the sleeve 9 is determined according to that of each groove to be made on the cylinder 8, the electrode 6 does not need to be moved according to the form of the groove. Therefore, the control of the electrolytic machining of the cylinder 8 is easy, and numerous grooves can be simultaneously made accurately in a short time.