A battery unit includes a case accommodating an electrode assembly, the case having an opening and a cap plate for covering the opening, the cap plate having a terminal insertion portion. The battery unit further includes a terminal member inserted into the case through the terminal insertion portion from an outside of the case and coupled to the electrode assembly, the terminal member including a current collector electrically coupled to the electrode assembly; a terminal portion extending parallel to an upper surface of the cap plate to an outside of the cap plate; and a connection portion electrically coupled to the current collector and to the terminal portion. The battery unit further includes a fixing member in the terminal insertion portion and fixing the terminal member to the cap plate, the fixing member including injection-molded plastic resin in the terminal insertion portion and surrounding the terminal member.