Cells are arranged in a predetermined arrangement and are held by holding unit. The holding unit includes a first holding unit, a second holding unit, and a third holding unit. The first holding unit holds cells so as to partially cover the one side of the outer peripheral surfaces of the cells in the longitudinal direction. The second holding unit holds the cells so as to partially cover the other side of the outer peripheral surfaces of the cells in the longitudinal direction. The third holding unit holds the cells so as to cover a region that is covered with neither the first holding unit nor the second holding unit, of the outer peripheral surfaces of the cells in the longitudinal direction.