An unauthorized connection detecting device which detects whether or not a power storage device that is connectable to a charge/discharge device for charging or discharging with power is an unauthorized power storage device, the unauthorized connection detecting device comprising: a communications unit configured to receive first charge/discharge information in which first identification information and first connection information are associated each other, the first identification information being information for identifying an encryption key of the power storage device used for mutual authentication between the charge/discharge device and the power storage device connectable to the charge/discharge device, and the first connection information being information on the power storage device and obtained when the power storage device is connected to the charge/discharge device; a control unit; a storage unit; and an unauthorization detecting unit configured to determine, using the first identification information and the first connection information included in the received first charge/discharge information, whether or not two or more power storage devices associated with a single first identification information item are present by reading through the control unit the first charge/discharge information stored in the storage unit, and detect that the power storage device connected to the charge/discharge device is the unauthorized power storage device if the two or more power storage devices associated with the single first identification information item are determined to be present. a communications unit configured to receive first charge/discharge information in which first identification information and first connection information are associated each other, the first identification information being information for identifying an encryption key of the power storage device used for mutual authentication between the charge/discharge device and the power storage device connectable to the charge/discharge device, and the first connection information being information on the power storage device and obtained when the power storage device is connected to the charge/discharge device; the first identification information being information for identifying an encryption key of the power storage device used for mutual authentication between the charge/discharge device and the power storage device connectable to the charge/discharge device, and the first connection information being information on the power storage device and obtained when the power storage device is connected to the charge/discharge device; a control unit; a storage unit; and an unauthorization detecting unit configured to determine, using the first identification information and the first connection information included in the received first charge/discharge information, whether or not two or more power storage devices associated with a single first identification information item are present by reading through the control unit the first charge/discharge information stored in the storage unit, and detect that the power storage device connected to the charge/discharge device is the unauthorized power storage device if the two or more power storage devices associated with the single first identification information item are determined to be present.