A heat-resisting ultrafine fibrous separator of the present invention is prepared by an electrospinning process, formed of ultrafine fibers of heat-resisting polymer resin having a melting point more than 1800 C or not having the melting point, or ultrafine fibers of polymer resin capable of swelling in an electrolyte, together with the ultrafine fibers of heat-resisting polymer resin. Also, polyolefine fine particles providing a shutdown function are dispersed in the heat-resisting resin or the polymer resin capable of swelling in the electrolyte. The heat-resisting ultrafine fibrous separator of the present invention has the shutdown function, low thermal contraction, thermal endurance, excellent ionic conductivity and excellent adhesive property with an electrode, so a battery having excellent cycling characteristics, and having high-energy density and high capacity can be prepared.