A polyester composition having high fireproofness is obtained by blending a copolymerized polyester comprising a polyalkylene terephthalate containing in the main chain thereof binding units of the formula: ##STR1## (WHEREIN, Y denotes -O-, an alkylidene radical having 1 to 6 carbon atoms or a cycloalkylidene radical having 3 to 6 carbon atoms) with a high molecular weight organic phosphorus compound of the formula: ##STR2## (WHEREIN, R.sub.1 denotes a monovalent hydrocarbon radical of 1 to 18 carbon atoms having or not having one or more substituents, R.sub.2 denotes a divalent hydrocarbon radical of 2 to 20 carbon atoms having or not having one or more substituents, x and z each denote 0 or 1 and n denotes an integer having the value of not less than 3) in an amount corresponding to 0.05 to 1.0% by weight of phosphorus atoms.