Disclosed herein is a cap assembly disposed on an open upper end part of a cylindrical can of a battery, the battery being configured to have an electrode assembly mounted in the cylindrical can, the cap assembly including a safety vent configured to rupture in order to exhaust gas when the interior pressure of the battery reaches a predetermined pressure limit, an upwardly-protruding cap plate disposed on the upper part of the safety vent, the cap plate having a through-opening configured to receive the gas exhausted therethrough, a current interrupt member attached to the lower end of the safety vent, the current interrupt member configured to interrupt electric current when the interior pressure of the battery reaches the predetermined pressure limit, and a guide member attached to the inside of the cap plate, the guide member being configured to prevent escape of a ruptured portion of the safety vent.