A secondary battery including an electrode assembly, a can accommodating the electrode assembly, a cap assembly sealing the can, and an insulating case interposed between the electrode assembly and the cap assembly and having two facing long side walls and two facing short side walls, wherein the two facing long side walls include opposing end portions and opposing central portions, the opposing end portions and the opposing central portions each include outer surfaces and a distance between the outer surfaces of the opposing end portions of the long side walls is greater than a distance between the outer surfaces of the opposing central portions of the long side walls.