A casing of sealed alkaline storage battery to be stacked in plural number in one direction, each provided with a safety vent; each casing is made of a polymer alloy which mainly comprises polyphenylene ether, polystyrene and an elastomer; at least one outersurface of the casing has a plurality of vertical protrusion ribs thereby to form vertical ventilation spaces through which a cooling medium passes; the vertical ribs formed on an outer surface of the casing have a protrusion height of 1 to 2 mm, the interval between ribs of 7 to 15 mm, rib width of 3 to 10 mm; thickness of inter-rib portion of casing are 1 to 3 mm, and those at the parts of short side walls 2 to 4 mm.