PURPOSE: To obtain a functional filament using an unburned fluorine-based resin filament and a heat resistant filament, a web using the functional filaments and a burned web obtained by burning the web, excellent in properties of heat resistance, fire-proof, chemical resistance, non adhesion, slipping, abrasion resistance, water-proof, electric insulation, etc.

CONSTITUTION: This functional filament consists of a covering filament (3A) having the structure of covering a heat resistant filament (1) with an unburned filament of a fluorine-based resin (2) or a functional filament (3) consisting of a twisted filament obtained by twisting the heat resistant filament (1) with the unburned filament (2) of the fluorine-based resin. A web is obtained by weaving the functional filament (3) used at least as a part, organizing and piling with arranging in parallel or in multiply crossing. The web may be coated with an unburned film of the fluorine-based resin, and these web are heat-treated to obtain a burned web.