PURPOSE: To improve accuracy on inner surface working by reducing an area of a grinding wheel contacting a workpiece in rough grinding to reduce the radial grinding resistance of a quill and the radial escapement of grinding wheel without tapering the workpiece.

CONSTITUTION: In rough grinding of large feed-in, the axis of a quill 16 is inclined by a required angle α° to the axis of a workpiece W to reduce an area of a grinding wheel 17 contacting a workpiece W. Thus, a grinding head is fed forward so that the grinding wheel 17 contacts the workpiece W on a small area, the radius grinding resistance of quill 16 is reduced and the radial escapement of grinding wheel 17 is reduced, while the inner surface of workpiece W is roughly ground without tapering. Then, the grinding stone 17 is withdrawn, the grinding head is reduced and turned and the axis of quill 16 is set parallel to the axis of workpiece W. Then, the grinding head is fed forward so that the grinding wheel 17 contacts the inner surface of workpiece W at a large area over the whole width for finish grinding.