When a voltage value of an electrical storage device becomes higher than a predetermined voltage value for identifying an overcharged state of the electrical storage device, calculation of a heat generation amount of the electrical storage device based on a current value is started. When the calculated heat generation amount is larger than a predetermined amount, it is determined that the electrical storage device is in an abnormal state (abnormal state related to heating). Because the electrical storage device enters an easily heated state when the voltage value becomes higher than the predetermined voltage value, the heat generation amount may be monitored by starting calculation of the heat generation amount. Once the voltage value becomes higher than the predetermined voltage value, irrespective of a high/low relationship between the voltage value and the predetermined voltage value, the abnormal state may be determined by monitoring the heat generation amount.