000 or more fiber can be easily made by electrospinning and the physical properties thereof are excellent further as the molecular weight thereof increases the diameter of the nanofiber obtained by electrospinning decreases thus forming a large number of junctions of nanofibers further the polymer layer 22 and the heat resistant resin layer 24 can be made of a polymer having a molecular weight of 2 000 or more in terms of workability and physical properties depending on mass production in particularly the polymer layer 22 and the heat resistant resin layer 24 can be made of ultra high molecular weight polyethylene having a molecular weight of 1 000 000 u02dc 5 000 000 if the solvent used to form the polymer layer 22 and the heat resistant resin layer 24 can suitably dissolve a polymer and disperse solid particles the solvent may be selectively used depending on the kind of polymer and solid particles by those skilled in the art the polymer layer 22 is made of a polyolefin resin or a polymer having a melt index of 1 u02dc 25 g min and a relatively low melting point such as polyethylene pe polypropylene pp polymethylpentene an ethylene propylene copolymer or the like further for the purpose of shutdown function the polymer layer 22 may be made of a polymer material having a melting point of 100 u02dc 180 u00b0 c preferably 120 u02dc 150 u00b0 c here a polyolefin resin or a polymer is used.