PURPOSE: To obtain the title fiber having both of fusion proofing property and flame retardance by irradiating a polyester fiber containing a specific amount or more of phosphorus compound with electron beam and then providing a specific amount of specific monomer to the fiber.

CONSTITUTION: A phosphorus compound, e.g. phosphorus-containing flame retardant material is melt and kneaded with a polyethylene terephthalate resin and the kneaded material is spun to afford 0.1-5wt.% of polyester fiber. Then the fiber is irradiated with electron beam and then a polymerizable monomer having ≤200 molecular weight, e.g. (meth) acrylic acid, etc., or esters thereof are provided to the fiber and the monomer is polymerized and fixed so as to increase the fiber weight by ≥7%. The fiber obtained by this treatment has fusion proofing performance and is excellent also in flame retardance.