Disclosed herein is a cap assembly disposed at an open upper end of a cylindrical container of a battery in which an electrode assembly (jelly roll) of a cathode/separator/anode structure is mounted in the cylindrical container, wherein an electrode lead of the jelly roll is electrically connected to the lower end of a safety vent having notches, which are ruptured to discharge high pressure gas when high pressure is generated in the battery, and a safety element to interrupt current at a predetermined temperature is provided between the safety vent and the electrode lead.