By a first energization process of applying a voltage with the power supply to cause a current for charging the electrical storage device to flow through the circuit and a second energization process of, when a transition condition is satisfied during the first energization process, decreasing the voltage of the power supply to cause the current to further flow, a condition of an electrical storage is determined. An effective resistance value of the circuit is set to 0.1Ω or below. A decrease in the voltage of the power supply in transition from the first energization process to the second energization process is set such that the effective resistance value in the second energization process is an intermediate value between a parasitic resistance value of the circuit and the effective resistance value in the first energization process.