Switches change the resistance value of a charge/discharge circuit in a period from starting a discharging at an upper limit voltage until the voltage reaches a lower limit voltage. An electric charge estimator computes electric charge by time-integrating current from a start of the discharging to an arbitrarily determined time, and computes a relationship between electric charge and voltage of a power storage device. An internal resistance estimator computes internal resistance based on voltages and currents of the storage device at times when resistance values are different. An electric energy estimator computes a relationship between electric charge and open voltage based on electric charge, voltage, current and internal resistance of the storage device. During charging or discharging of the storage device, the electric energy estimator estimates the electric energy of the power storage device based on the electric charge, the open voltage, the internal resistance, and the charge/discharge current.