A control device of a secondary battery uses, as material of a positive electrode, positive electrode active material that shows a difference of an open circuit voltage curve between charging and discharging. A storing unit stores, as discharge open circuit voltage information, a relationship between an SOC in a discharge process and an open circuit voltage for each changeover SOC that is an SOC when changing a state of the secondary battery from the charge to the discharge. An SOC calculating unit calculates the SOC of the secondary battery in the discharge process on the basis of a changeover SOC when actually performing the change from the charge to the discharge and the discharge open circuit voltage information stored in the storing unit. It is therefore possible to properly detect the SOC during the discharge.