A cell module including a plurality of flat battery cells (1) to be stacked; and a casing (3) in which the battery cells are accommodated, wherein the casing comprises a container-shaped lower part (30) having an engaging claw (30e) on a side wall portion (30c, 30d), and an upper part (31) in which the battery cells are disposed between the lower part and the upper part and having an engaging hole (31e) with which the engaging claw of the lower part is engaged.