PURPOSE: To assure the synchronization of a reference motor and an another motor when an electric power source or load fluctuates by sampling new speed information when the electric current of the reference motor fluctuates within the sampling time of speed information.

CONSTITUTION: An electric current detecting circuit 14 is provided in the rear stage of a driving circuit 7, and the electric current of a reference motor 4 is detected by the circuit 14. An optical system control circuit 13 samples the speed information from an encoder 9 in accordance with the command from a main control circuit 8, holds the same in a memory, determines a reference speed and applies a reference clock to a motor control circuit 11. When the current of the motor 4 is not within the predetermined range, the circuit 13 cancels the speed information sampled previously and held in the memory, samples and holds the speed information from the encoder 9 newly at another timing, determines the reference clock to the circuit 11 and determines the speed reference of a DC servomotor 6. Thus the motors 4 and 6 are synchronized without fail.