PURPOSE: To work plural types of gears having different tooth ratio efficiently, by connecting a selective output device for outputting the pulse selectively to a pulse generator for synchronizing the rotations of a work and a tool.

CONSTITUTION: A pulse generator 4 coupled to a grindstone 3 will produce the pulse number n4 in accordance with the rotation. A selective pulse output device 5 will produce the predetermined pulse number n18 which is Kn times of the pulse number n4 in order to synchronize the rotations of a work 17 and a grindstone 3. A work motor 11 will rotate the work 17 through gears 13, 14. The pulse number n18 is selected to the predetermined number to be determined by the tooth ratio of the work 17 and can be set by a selective pulse output device 5.