A cell assembly (20) is formed by arranging an electrode body and electrolyte in an external case (12) having a through hole (41). A laser is applied to an annular portion (formed by Al or Al-based alloy) surrounding an opening end (41A) of the through hole (41) toward outside the case so as to form a welded base (45). A sealing film (54) is welded to the surface of the welded base directly or via a base film (52) welded in advance, so as to temporarily seal the through hole (41). The cell assembly (20) in the temporarily sealed state is subjected to initial charge and then a hole is opened through the film (54) so as to form a gas discharge passage for communication between inside and outside of the case (12), thereby discharging an excessive gas from the case (12) outside.