SECONDARY BATTERY

A secondary battery includes a cylindrical case, an electrode assembly accommodated in the cylindrical case, and a cap plate electrically connected to the electrode assembly and configured to seal the cylindrical case, wherein the cap plate includes a first flat portion at a center thereof, a second flat portion outside the first flat portion, and a connecting portion between the first flat portion and the second flat portion, and wherein a height of a top surface of the first flat portion is higher than a height of a top surface of the second flat portion.