To prevent the emission of fluorescence from the subject polymer and obtain a forming free from fluorescence-caused deterioration of beautiful appearance by copolymerizing a polyalkylene naphthalate with a dicarboxylic acid component containing a phenylquinoxa1ine.

A polyalkylene naphthalate, in which at least 80mol% of a constituting unit is composed of a recurring unit of formula I (R is a 2-6C alkylene) [preferably, poly(1,2-ethylene-2,6-naphthalenedicarboxylate)], is copolymerized with a dicarboxylic acid component containing a phenylqunoxaline of formula II [R1 is H, a 1-6C alkyl, a halogen; (m)=1-4] [e.g. 2,3-bis(4-carboxyphenyl)quinoxaline] in an amount of 0.01-5mol%, preferably 0.3-3mol% based on the total of dicarboxylic acid components. Further, the polyalkylene naphthalate has preferably an intrinsic viscosity of ≥0.5 at 35°C in a mixed solvent of phenol/1,1,2,2-tetrachloroethane (60/40 in weight ratio).