An auxiliary battery storage device stores an auxiliary battery that is used for performing cell balancing to a main battery adopted in an electric vehicle or supplying power to the main battery having low state of charge (SOC). The auxiliary battery storage device includes a case body formed in a box shape with a top portion that is open and an inner space in which the auxiliary battery is stored; a case body cover coupled to the case body to open or close the top portion of the case body; a terminal connection member fixed to an inner surface of the case body cover to contact a power terminal of the auxiliary battery when the top portion of the case body is closed by the case body cover; and a power cable configured to extend from the terminal connection member to the main battery.