An alternating current generation system is provided. In the system, in a rotor, a plurality of detection subject portions are arrayed along a circumferential direction in correspondence to reversal states of the magnetic poles in a plurality of magnetized portions. A detecting unit is disposed opposing the rotor so as to generate an output signal corresponding to passage state of the detection subject portions. A phase control unit outputs, to a power converter, a control signal to perform phase control of switching elements depending on the rotation phase of the rotor, based on the output signal from the detecting unit.