To perform grinding work with stable sharpness by keeping film thickness of electrolytic film on a surface of a grinding wheel constant.

A feeder 2 is brought to get in contact with the grinding wheel 1, an electrode 3 is put to face the surface 1a of the grinding wheel, and a voltage is applied to form the electrolytic film. For keeping the film thickness of the electrolytic film constant to maintain sharpness, the film thickness of the electrolytic film is determined based on output of an eddy current sensor 6 to detect distance to a magnetic substance as base material of the grinding wheel 1, and output of a contactless type displacement sensor 7 to detect position change of the surface grinding wheel 1a, and electrolytic conditions are controlled, whereby fluctuation of a film thickness value is prevented.