PURPOSE: To speedily acquire a correct phase by correcting the quantity of feedback to a control system for the phase difference error quantity of a speed reduction side corresponding to external load variation when a cylinder driving circuit which is not equipped with a speed reduction driving function is used.

CONSTITUTION: The difference between the phase difference of a PG signal from a reference rotation signal 20 and a phase difference target value T is found by a phase difference detecting circuit 10 and a subtracter 11 and inputted to a phase difference error converting circuit 12. The phase difference error converting circuit 12 multiplies the phase difference error by a constant value corresponding to the external load placed on a rotary cylinder 3 and outputs the result to an adder 14 and the phase difference error is fed back to a control system.