To provide a polyester composition having high flame-retardancy, and to provide a fiber structure comprising the composition.

The polyester resin composition comprises components (A), (B), (C) and (D): (A) 40-90 wt.% polyester resin of which 80 mol% is composed of an ethylene terephthalate unit; (B) 3-20 wt.% one or more selected from an ammonium polyphosphate having heat loss at 300C of 5 wt.% and a metal phosphate; (C) 3-20 wt.% compound having heat loss at 300C of 5 wt.% and containing at least one or more nitrogen atoms in the molecule; and (D) 0.5-20 wt.% organopolysiloxane resin represented by general formula (1): (C6H5)m1Rm2SiXnO(4-m1-m2-n)/2, wherein R is a hydrogen or a monovalent organic group except a phenyl group; X is an OH group or a hydrolyzable group; m1, m2, and n are each a number satisfying the following expressions: 0.3m11.8; 0m21.5; 1.0m1+m22.0; 0.2m1/(m1+m2)1.0; 0n1.5; and 1.0m1+m2+n3.0.