A battery pack includes an outer case, and a plurality of cylindrical lithium ion secondary batteries accommodated in the outer case in vertical N rows and lateral M rows (N rowsM rows), with a gap between the secondary batteries, wherein the outer case has a draft port opened in a first wall facing an outer circumferential surface of the secondary batteries in the first vertical row, and an exhaust port opened in a second wall positioned at the opposite side of the first wall, in each secondary battery in the first vertical row and the second vertical row, a part of the outer circumferential surface, which is at a shorter distance from the first wall than any other parts, faces the draft port of the outer case.