A sealed rectangular battery having a reliably operative explosion-proof valve is provided. A rectangular battery case 1 whose cross-section is oblong, the battery case having a prismatic shaped case body 5 with a bottom and a lid 6. An explosion-proof valve 2 is arranged near one of two opposing short sides of a main surface wall of the lid. The explosion-proof valve a V-cut groove 15 concavely formed in V-shaped or U-shaped in which a central bent part 15a is positioned at a central location in the main surface wall 6a of the lid, and a thin wall 16 formed by the V-cut groove. Cut grooves 18 connecting each of two ends of the V-cut groove and each of two long sides of the main surface wall of the lid which are parallel to each other, respectively, are cut and formed in the main surface wall of the lid.