Sealed rectangular battery

A sealed rectangular battery having a reliably operative explosion-proof valve is provided. A rectangular battery case 1 which is long in the horizontal direction includes a square tube shaped case body 5 with a bottom in which the upper surface opens, and a lid 6 closing the opened upper surface of the case body 5 in a sealing manner. An explosion-proof valve 2 is arranged near a reference short side 6c of a installation surface wall 6a of the lid 6. The explosion-proof valve 2 includes a V-shaped 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 front-back direction of the installation surface wall 6a and expanding in the front-back direction, and a thin wall 16 formed by the V-shaped groove 15. A cut groove 18 connecting the front-back end of the V-shaped groove 15 and the long side 6d in the front-back direction of the installation surface wall 6a is cut and formed in the installation surface wall 6a.