In attaching a cover, the cover is attached to a placement jig such that positioning pins of the placement jig are respectively passed through a first bolt insertion portion and a second bolt insertion portion of the cover. In placing the cover, the cover is placed at a placement position on a first surface of a storage body such that the positioning pins of the placement jig to which the cover is attached are respectively inserted into a first fastening hole and a second fastening hole of the storage body.