Patent ID: 8297035

Claim:
A copolymerized polyester having a main repeating unit comprising ethylene terephthalate, the copolymerized polyester comprising, in an acid component forming the copolymerized polyester, a metal salt of sulfoisophthalic acid (A) and a compound (B) expressed as the following formula (I) in such a state as to simultaneously satisfy the following mathematical expressions (1) and (2), wherein in the formula, R represents a hydrogen atom, or an alkyl group having 1 to 10 carbon atoms, X represents a quaternary phosphonium ion or a quaternary ammonium ion; 3.0 ≦A+B≦ 5.0 (1) 0.3 ≦B /( A+B )≦0.7 (2) where in the mathematical expressions, A represents the copolymerization amount (mol %) of the metal salt of sulfoisophthalic acid (A) based on the total amount of acid components forming the copolymerized polyester, and B represents the copolymerization amount (mol %) of the compound (B) expressed as the formula (I) based on the total amount of acid components forming the copolymerized polyester.