PURPOSE: To prevent the run-off of the titled filter in a period of use by heating the blended yarn of both copolymerized polyester fiber of specified composition and polyester fiber having m.p. lower than this fiber, molding the blended yarn and fusing one part of the copolymerized polyester fiber.

CONSTITUTION: 50W95wt% copolymerized polyester staple fiber contg. sulfoisophthalic acid or its metallic salt and 5W50wt% polyester staple fiber having m.p. by ≥50°C lower than the copolymerized polyester are fed to a fiber blending machine and the sliver wherein both the two are uniformly blended is produced. The sliver is uniformly packed between the perforated covers having about five or more pieces of holes of about 0.1W0.2mm diameter and the inside of cylinder is vacuumed in a closed state. Then the heated air higher by about 20°C than the low m.p. polyester is blown thereinto to fuse the low m.p. polyester and the fiber structure stuck with one part of the copolymerized polyester fiber is formed.