The present invention relates to a cap assembly which is mounted on an open upper end of a cylindrical can in a battery structure in which an electrode assembly (jelly-roll) having a positive electrode/separator/negative electrode structure is built in the cylindrical can. The cap assembly comprises: a safety vent forming a notch which is fixed such that the safety vent may be broken by a high-pressure gas from the battery; a protruding upper-end cap connected along an outer surface of the safety vent; a gasket mounted on an outer surface of the upper cap; and a fire-extinguishing member comprising fire-extinguishing components for preventing the battery from igniting, and having at least one portion mounted at the interface between the upper cap and the gasket.