A secondary battery according to the present invention including a battery case with an accommodation part accommodating an electrode assembly and an electrolyte, a fluid inlet/outlet port provided in the battery case to provide a moving passage through which a fluid is movable so that discharge of an inner gas and reinjection of the electrolyte are enabled, a gas discharge part restricting the discharge of the inner gas passing through the fluid inlet/outlet port, and an electrolyte reinjection part provided to reinject the electrolyte through the fluid inlet/outlet port.