PURPOSE: To efficiently form a mirror surface uniformly in roughness of 0.2μm Rmax or less, by combining the electrolytic action, which melts the metal in a metallic work of positive polarity to be removed, with the mechanical action by abrasive grains.

CONSTITUTION: A disc-shaped electrode 10 of negative polarity is mounted to the bottom end face of a base part 7, and the central part of said electrode is formed into a hollowed part 11 or a hole part. An insulating water-permeable abrasive 13 is mounted to the bottom surface of the electrode 10, machining a metallic work 14 of positive polarity. Using an abrasive grain in a size of #800W #3,000, if assuming D (mm) for diameter of a tool surface, P (kgf/cm2) for pressing pressure of a grinding tool to the work, V (mm/min) for relative moving speed between the grinding tool and the work and N (rpm) for number of revolution of the grinding tool, their relation is taken as V/(DN3√P)<0.95 (kgf/cm2)-1/3.