In a secondary battery production method that has a gas discharge process that discharges gas from apertures (AP) formed in the casing (BC) of the secondary battery, the discharge of internal gas from the apertures (AP) when the internal pressure within the casing (BC) is greater than the external pressure is permitted in order to suppress the occurrence of battery bulges in the secondary battery while suppressing the rise in production costs as much as possible. In addition, a first seal (FS) that is displaced or deformed by the pressure difference between the inside and the outside of the casing (BC) is fitted in such a manner that the admission of outside air through the aperture (AP), when the internal pressure within the casing (BC) is lower than the external pressure, is prevented. The casing (BC) to which the first seal (FS) is fitted is placed in a reduced pressure environment and the gas within the casing (BC) is allowed to flow out of the casing (BC) through the points where the first seal (FS) is fitted. Then the casing (BC) is returned to a normal pressure environment and the apertures (AP) are sealed.