A control device of a secondary battery using, as positive electrode material, positive electrode active material showing a difference of an open circuit voltage curve between during charge and discharge, detects whether the secondary battery is in a charge state or a discharge state. When the secondary battery is in the charge state, the control device calculates, as a current SOC, a value that is lower than an SOC corresponding to a current open circuit voltage of the secondary battery on a reference SOC-open circuit voltage curve showing a relationship between the SOC and the open circuit voltage, which becomes a reference upon calculating the current SOC. When the secondary battery is in the discharge state, the control device calculates as the current SOC, a value that is higher than the SOC corresponding to the current open circuit voltage of the secondary battery on the reference SOC-open circuit voltage curve.