A secondary battery includes an electrode assembly; a case that accommodates the electrode assembly; and a cap assembly that is coupled to an upper portion of the case, the cap assembly including a cap up, a safety vent below the cap up, and a cap down below the safety vent and electrically connected to the electrode assembly, wherein the safety vent includes a main notch and a sub-notch adjacent to the main notch, and the sub-notch has a depth that is smaller than a depth of the main notch.