To improve the surface roughness of a workpiece by inclining the rotation shaft of a grinding wheel whose axial section is formed of a circular arc to the rotation shaft of the workpiece in grinding the inner or outer surface of the workpiece into a groove shape by use of the grinding wheel.

A workpiece 1 is cylindrical and rotatable around a rotational shaft 3, and the inner surface thereof is ground. A grinding wheel 2 is mounted to a tip of a rotary shaft 4 and has a circular arc shape in its axial section. The rotational axis 4 is inclined at the angle θ to the rotational shaft 3 in a plane including the shaft 3. In the grinding operation, the line of abrasive grains 5 at the outer circumferential surface of the grinding wheel 2 is inclined at the angle 5 to the inner circumferential direction of the workpiece 1. Each grain 5 in the line of grains 5 on the outer circumferential surface of the workpiece 1 acts on the circumferential line at the different position in the inner circumferential surface of the workpiece 1. As a result, the inner circumferential surface of the workpiece 1 is ground by the grains having an averaged pitch (b) smaller than that (a) of the line of the grains 5, and thereby the surface roughness is improved.