In a system for controlling rotation of a rotor of a multiphase rotary electric machine in relation to a stator thereof, a superimposing unit superimposes a first frequency signal on the input signal to the multiphase rotary electric machine. The first frequency signal has a first phase and a first period, and the first period is different from a period of rotation of the rotor. An amplitude detector detects an amplitude of a second frequency signal. The second frequency signal is actually propagated in the multiphase rotary electric machine with a second phase based on the superimposed first frequency signal. A rotation angle determiner determines a rotation angle of the rotor so as to eliminate a difference between the detected amplitude of the second frequency signal and a predetermined target amplitude thereof.