Disclosed are a method for manufacturing a secondary battery and an auxiliary case configured for use during manufacturing of the secondary battery. A gas generated in the secondary battery may be discharged to prevent an electrode assembly and the secondary battery from increasing in thickness, and an electrolyte may be efficiently injected into the secondary battery. A method for manufacturing a secondary battery includes: an accommodation step of accommodating an electrode assembly into a battery case; a connection step of connecting an auxiliary case to the battery case, the auxiliary case having an inner space therein; an injection step of injecting an electrolyte into the battery case and the auxiliary case; an electrolyte supply step of moving some of the electrolyte from the auxiliary case into the battery case; and a gas supply step of moving a gas from the battery case into the auxiliary case.