PURPOSE: To grind the tooth surface highly accurately by detecting whether the grinding surface of conical grinding wheel is matched with the generating bus of the tooth surface of cone warm gear and performing position correcting motion of grinding wheel.

CONSTITUTION: First movable table 10 on fixed table 18 can be moved in the axial direction S of cone warm gear 1 through a driver 19 while second movable table 11 on table 10 can be moved in perpendicular direction against the axis S through a driver 20 and rotary table 12 on table 11 can rotate around a rotary axis. The conical grinding wheel 14 fixed at the end of grinding wheel holder 13 on the table 12 is formed such that the conical angle will match with the pressure angle (α) of the tooth 1A of gear 1. The detecting section 15a in position detector 15 in a table 21 movable in parallel with the axis S will produce a signal when not matching with the generating bus L of grinding face 14a and a controller 16 will command position correcting motion to the tables 10, 11 and 12 such that the grinding face 14a will contact with the detecting section 15a.