A cylindrical battery gasket for sealing the opening of a battery canister, of a cylindrical battery further comprising a ring-shaped positive electrode mixture and a cylindrical separator with a bottom inserted into an inner side of the ring-shaped positive electrode mixture, wherein the gasket comprises: a boss part with a central hole through which a negative-electrode collector is inserted, and a canister-contact part for touching an inner side of the opening, upon the battery canister being sealed, a disk-shaped part extending radially from an outer periphery of the boss part, connecting the boss part to the canister-contact part, and a bent stress-buffering part, along the way to the disk-shaped part which absorbs the radial distortion of the gasket upon the battery canister being sealed, the stress-buffering part including two or more sharply-angled bent parts (37a, 37b), and being nearer a center of the battery than to the positive electrode mixture, upon the battery canister being sealed by the gasket, the two or more sharply-angled bent parts further comprising a first convexly formed bent part and a second concavely formed bent part, which is nearer an outer periphery of the gasket than the first bent part, wherein the first and second bent parts (37a, 37b) are connected by a reinforced section, which extends between the first and second bent parts (37a, 37b) and has a thickness that is greater than that of the first bent part nearer the center of the gasket and of the second bent part on the outer periphery of the gasket, and wherein a first section connecting the boss part and the first bent part is slanted downwardly in a direction from the boss part of the gasket towards the outer periphery of the gasket, the reinforced section extending from the first bent part towards the second bent part is slanted upwardly in a direction from the boss part of the gasket toward the outer periphery of the gasket, and a second section extending from the second bent part and connecting the second bent part and the canister-contact part is slanted downwardly in a direction from the boss part of the gasket to the outer periphery of the gasket, the second section also having an area cross-sectionally tapered that expands towards the outer periphery of the gasket. a boss part with a central hole through which a negative-electrode collector is inserted, and a canister-contact part for touching an inner side of the opening, upon the battery canister being sealed, a disk-shaped part extending radially from an outer periphery of the boss part, connecting the boss part to the canister-contact part, and a bent stress-buffering part, along the way to the disk-shaped part which absorbs the radial distortion of the gasket upon the battery canister being sealed, the stress-buffering part including two or more sharply-angled bent parts (37a, 37b), and being nearer a center of the battery than to the positive electrode mixture, upon the battery canister being sealed by the gasket, the two or more sharply-angled bent parts further comprising a first convexly formed bent part and a second concavely formed bent part, which is nearer an outer periphery of the gasket than the first bent part, wherein the first and second bent parts (37a, 37b) are connected by a reinforced section, which extends between the first and second bent parts (37a, 37b) and has a thickness that is greater than that of the first bent part nearer the center of the gasket and of the second bent part on the outer periphery of the gasket, and wherein a first section connecting the boss part and the first bent part is slanted downwardly in a direction from the boss part of the gasket towards the outer periphery of the gasket, the reinforced section extending from the first bent part towards the second bent part is slanted upwardly in a direction from the boss part of the gasket toward the outer periphery of the gasket, and a second section extending from the second bent part and connecting the second bent part and the canister-contact part is slanted downwardly in a direction from the boss part of the gasket to the outer periphery of the gasket, the second section also having an area cross-sectionally tapered that expands towards the outer periphery of the gasket.