In a battery case sealing method, a lid is fitted to an opening of a battery case, and then a side wall on an inner side of the battery case and a side surface of the lid are brought into contact with each other. The side wall forms the opening. In a state where the side wall on the inner side of the battery case is in contact with the side surface of the lid, a boundary portion between the side wall and the side surface of the lid is welded by irradiating a laser beam in a direction from an outer side of the battery case toward the inner side of the battery case in a thickness direction of the lid.