PURPOSE: To raise the processing speed to a great extent while maintaining a certain processing accuracy without causing complication of the whole device by furnishing No.1, No.2 control means which control No.1, No.2 rotating means on the basis of phase difference signals.

CONSTITUTION: When the phase difference (f) between a grinder gear 10 and a work gear 12 enlarges, the phase difference voltage emitted from a phase comparator 22 itself becomes greater than before, and the amount corresponding to voltage drop due to a phase difference setting device 28 being set to the neg. potential side will be compensated. Accordingly the phase difference voltage entered in No.1 control means 30 recovers the previous size, and the sum voltage with a constant voltage from a speed setting device 32 of the No.1 control means 30 will again become of the same size as the constant voltage from a speed setting device 36 of No.2 control means. That is, the output voltages of amplifiers 31, 35 become equal, and servo motors 16, 14 rotate the above- mentioned work gear 21 and grinder gear 10 again at the specified constant speed.