A flame resistant polyester produced by addition and copolymerization of a phosphorus compound represented by general formula (I) below as a diol component of the polyester in an amount of 500 ppm or more in terms of phosphorus atom content in the polyester, which polyester has alkylene terephthalate units as main repeating units and which has an intrinsic viscosity of not less than 0.5 measured at 20.degree. C. in a mixture of equal weight of phenol and tetrachloroethane: ##STR1## where E is --CH.sub.2 CH.sub.2 --, Ar is an aromatic group or an aromatic group substituted by a lower alkyl group or a halogen atom, and m and n denote, respectively, integers the sum of which is from 1 to 20.