A secondary battery comprising: a battery case comprising 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; an electrolyte reinjection part provided to reinject the electrolyte through the fluid inlet/outlet port, and a gas filter part provided on the moving passage to be close to the fluid inlet/outlet port, thereby filtering harmful components when the inner gas is discharge, wherein the gas discharge part comprises: a sealing cover (141; 142) covering and sealing the moving passage; and an elastic member applying elastic force to the sealing cover to cover the moving passage, wherein, when the inner gas has a pressure greater than the elastic force of the elastic member, the moving passage covered by the sealing cover (141; 142) is opened to discharge the inner gas to the outside of the battery case, the fluid inlet/outlet port is provided on each of both ends of the battery case, the sealing cover (141; 142) is provided in a pair to respectively cover and seal the moving passages provided in both the ends of the battery case, the elastic member is provided between the pair of sealing covers (141; 142) to apply the elastic force in a direction in which the pair of sealing covers (141; 142) are closer to each other, the fluid inlet/outlet port has a discharge hole forming a passage connected to the moving passage, and the sealing cover (141; 142) of the gas discharge part covers an outer surface of the fluid inlet/outlet port having the discharge hole to seal the discharge hole, thereby sealing the moving passage. a battery case comprising 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; an electrolyte reinjection part provided to reinject the electrolyte through the fluid inlet/outlet port, and a gas filter part provided on the moving passage to be close to the fluid inlet/outlet port, thereby filtering harmful components when the inner gas is discharge, wherein the gas discharge part comprises: a sealing cover (141; 142) covering and sealing the moving passage; and an elastic member applying elastic force to the sealing cover to cover the moving passage, wherein, when the inner gas has a pressure greater than the elastic force of the elastic member, the moving passage covered by the sealing cover (141; 142) is opened to discharge the inner gas to the outside of the battery case, the fluid inlet/outlet port is provided on each of both ends of the battery case, the sealing cover (141; 142) is provided in a pair to respectively cover and seal the moving passages provided in both the ends of the battery case, the elastic member is provided between the pair of sealing covers (141; 142) to apply the elastic force in a direction in which the pair of sealing covers (141; 142) are closer to each other, the fluid inlet/outlet port has a discharge hole forming a passage connected to the moving passage, and the sealing cover (141; 142) of the gas discharge part covers an outer surface of the fluid inlet/outlet port having the discharge hole to seal the discharge hole, thereby sealing the moving passage. a sealing cover (141; 142) covering and sealing the moving passage; and an elastic member applying elastic force to the sealing cover to cover the moving passage, wherein, when the inner gas has a pressure greater than the elastic force of the elastic member, the moving passage covered by the sealing cover (141; 142) is opened to discharge the inner gas to the outside of the battery case, the fluid inlet/outlet port is provided on each of both ends of the battery case, the sealing cover (141; 142) is provided in a pair to respectively cover and seal the moving passages provided in both the ends of the battery case, the elastic member is provided between the pair of sealing covers (141; 142) to apply the elastic force in a direction in which the pair of sealing covers (141; 142) are closer to each other, the fluid inlet/outlet port has a discharge hole forming a passage connected to the moving passage, and the sealing cover (141; 142) of the gas discharge part covers an outer surface of the fluid inlet/outlet port having the discharge hole to seal the discharge hole, thereby sealing the moving passage.