A switching element, a duty deciding circuit, an LRC circuit, a voltage detecting circuit, and an LRC limit value deciding circuit are included. The switching element 58 outputs a duty factor of a field winding of a vehicle power generator towards an ECU, via an FR terminal. The duty deciding circuit limits a speed at which the duty factor of the field winding increases to a predetermined limit value. The voltage detecting circuit detects a voltage at the FR terminal. The LRC limit value deciding circuit switches the limit value of the speed at which the duty factor increases based on the voltage value detected by the voltage detecting circuit.