PURPOSE: To shorten converging time under synchronization mode and to prevent application of unnecessary torque impact onto a sample by forcibly bringing the phase difference and the differential rotation command to zero when the phase difference exceeds a set value and the differential rotation goes to zero.

CONSTITUTION: When a value corresponding to the area upto time t1 exceeds a set value KA, a phase difference comparator 22 produces an inverted output. The inverted output is sustained until the output A from an up/down counter 18 drops below KA. When the differential rotation N goes to zero at time t2, a zero differential rotation detector 23 produces an output. An AND gate 24 produces a reset output to reset the up/down counter 18 thus bringing the output from a microcomputer 19 to zero. Output from a D/A converter 20, i.e., the differential rotation command N, is also brought to zero. Regulation of the differential rotation between both motors is stopped and synchronization control resumes with the differential rotation N=0 while keeping the phase difference. Since the operation is continued under forcibly synchronized state, converging time is shortened.