To improve grinding efficiency without lowering machining accuracy by machining while changing the rotating direction of a grinding wheel normally or reversely at optional intervals in a method of machining the outer spherical surface by the rotation of the grinding wheel, the rotation of a workpiece and the oscillation of the grinding wheel along the spherical surface of the workpiece.

A workpiece W is held to a workpiece fitting shaft 2, and an annular grinding wheel 1 is brought into contact with the spherical surface from one side to perform grinding. At this time, the workpiece W is rotated in one direction by a motor 3, and the grinding wheel 1 is rotated around a grinding wheel shaft 4 as a rotary shaft and also oscillated along the spherical surface of the workpiece W. The grinding wheel 1 is also put in repetitive action of normal and reverse rotation at intervals preset by a timer, and the grinding wheel 1 is oscillated by oscillating a rotary plate 6 with a grinding wheel driving part 5 fixed thereto, around a rotary shaft 7. With the grinding wheel 1 thus rotated both normally and reversely, the generation of difference to a grinding degree is prevented, and a new grinding track is created so as to attain more efficient grinding.