From time (t1) at which an IG switch is turned off, charging/discharging of a secondary battery is stopped. Polarization cancellation time (Tr) required for cancelling polarization after stopping charging/discharging is changed in accordance with a temperature of the secondary battery. Until an elapse of a predetermined time corresponding to polarization cancellation time (Tr) from the time of turning off the IG switch, the battery voltage is detected cyclically, and an open circuit voltage is sequentially estimated in the equilibrium state in which the polarization is cancelled based on a shift of the battery voltage. In a case where the IG switch is turned on after an elapse of the predetermined time from the time of turning off the IG switch, a detection value of the battery voltage at the time of IG-on is assumed to be the open circuit voltage, and then the SOC at the time of starting use of the secondary battery is estimated. In a case where the IG switch is turned on before an elapse of the predetermined time from the time of turning off the IG switch, the estimated value of the opening circuit voltage at that time point is regarded as the open circuit voltage, and the SOC at the time of starting use of the secondary battery is estimated.