A secondary battery includes a can having an opening at one end, an electrode assembly accommodated in the can through the opening, a cap plate that seals the opening, and an insulation plate below the cap plate. The can includes: a first surface that is convex; a second surface that faces the first surface and is concave; a third surface that is curved and connected to the first surface and the second surface to form a bottom surface; a first connection portion that connects the first surface and the third surface and has a round form having a first radius of curvature; and a second connection portion that connects the second surface and the third surface and has a round form having a second radius of curvature. At least one of the first radius of curvature and the second radius of curvature varies along a length of the third surface.