PURPOSE: To maintain desirable contact characteristics between meshed teeth over a wide range load and an installing adjustment by reciprocatively displacing a workpiece along the cradle axis by oscillation of a grinding wheel and the timed relationship.

CONSTITUTION: A grinding wheel 26 contains the inner material removing surface 148 and the outer material removing surface 150 on the periphery to work the side part of adjacent tooth, so that a worked gear 20 is pulled in by a distance (n) to the grinding wheel 26 when longitudinally oscillating around the cradle axis 66 from a central position of the teeth. While an instantaneous radius 152 of curvature of the recessed surface of the worked gear 20 measured in the pitch surface 160 of the worked gear 20 is larger than an instantaneous radius 154 of curvature in the same pitch surface, an instantaneous radius 156 of curvature of the projecting surface of the worked gear 20 is smaller than an instantaneous radius 158 of curvature, and since the work gear 20 is pulled in by the distance (n), while the radius of curvature of the recessed surface reduces, the redius of curvature of the projecting surface increases. Therefore, desirable contact of the expected tooth surface can be obtained when a position of the worked gear 20 running along the cradle axis 66 is controlled to an angle position of a pan-shaped grinding wheel 26 around the cradle axis 66.