A battery which includes a can having an opening at one side thereof, an electrode assembly contained in the can, a cap plate closing the opening and including an electrode pin at an upper surface thereof and an anchor protruding outwards from a bottom surface thereof; and an insulating plate under the cap plate, wherein the cap plate includes a first side surface that is convex and a second side surface that is concave; the insulating plate has at a first end thereof an anchor opening combined with the anchor; and a width of the first end of the insulating plate is greater than a width of a second end of the insulating plate.