A grinding wheel having a convex grinding surface is attached to a wheel spindle that is contactlessly supported by a casing through a controlled type axial magnetic bearing and controlled type radial magnetic bearings and that is rotated by an electric motor. A groove of a workpiece is ground by causing relative displacement of the casing. After the axial positioning of the casing is performed, the casing is radially displaced. Then, the groove is ground. The stiffness value of the axial magnetic bearing 6 is set to be lower than a normal value before the casing is radially displaced. After it is detected that the grinding surface of the grinding wheel touches the entire groove of the workpiece and that the grinding of the entire surface of the workpiece is started, the stiffness value is set back to the normal value.