In a vacuum container (2), a bag-shaped laminate film (12) containing a battery element (11) and having an opening (12a) is pinched at positions corresponding to two principal surfaces (11a) of the battery element (11), the battery element (11) having a positive layer and a negative layer stacked via a separator. Pressure in the vacuum container (2) is reduced. An electrolytic solution (20) is poured from an electrolytic-solution supply line (4) into the bag-shaped laminate film (12) through the opening (12a) with pressure in the vacuum container (2) kept reduced until the entire battery element (11) is immersed in the electrolytic solution (20). The reduced pressure in the vacuum container (2) is increased to make the battery element (11) absorb the electrolytic solution (20) by means of the difference in pressure.