An alternator 1 connected to a battery mounted in a vehicle, a regulator 20 including a regulator IC 4 for adjusting a power generation voltage of the alternator 1, and an ECU connected to the regulator 20 are provided. A DF signal is inputted from the regulator 20 to the ECU 10. An average value obtained by performing an averaging process an ON time of the DF signal measured during predetermined sampling time is used as ON ratio information of the DF signal for reduction of a circuit size, optimization of the circuit, control stabilization, and cost reduction.