PURPOSE: To obtain a prescribed inclination angle of a grinding tool which continuously changes to a dressing tool, and assure functionality by dressing the grinding tool by surface contact between the dressing tool and the grinding tool, and electronic forced guide of rolling and relative motion between these tools.

CONSTITUTION: A tooth flank of a gear-shaped workpiece is processed by an internally toothed grinding tool. Dressing the grinding tool by a gear-shaped dressing tool after this processing is conducted by one tooth flank contact or two tooth flank contact between a dressing tool and the grinding tool, and electronic forced guide of rolling and relative motion between the dressing tool and the grinding tool. As a result, a head radius of the grinding tool is enlarged for R, and a contact zone is moved to (A-D)1a, (A-D)2a. After that, by free or torque-guided two-flank engagement between the grinding tool and the workpiece after dressing, the gear-shaped workpiece is precision worked.