To obtain a polyester having excellent gas barrier properties, transparency, etc., slightly generating acetaldehyde, comprising a specific dicarboxylic acid constituent unit and a prescribed dial constituent unit by specifying an intrinsic viscosity, melting point, etc.
This polyester comprises (A) a dicarboxylic acid constituent unit composed of 40-99.5 wt.% based on the whole dicarboxylic acid constituent unit of a constituent unit from terephthalic acid, 0.5-60 wt.% of a constituent unit from a naphthalenedicarboxylic acid and 0-25 wt.% of a constituent unit from isophthalic acid and (B) a dial constituent unit composed of 95-100 wt.% based on the whole diol constituent unit of a constituent unit from ethylene glycol and 0-5 wt.% of a polyalkylene glycol constituent unit containing a 2-10C alkylene oxide chain, has 0.5-1.5 dL/g intrinsic viscosity and a melting point (Tm) satisfying the equation [1/527-0.0017.ln(1-(mI+mN)/200)]-1-273<Tm≤254 [mI is the ratio (mol.%) of a constituent unit from isophthalic acid in the total dicarboxylic acid constituent unit; mN is the ratio of a constituent unit from the naphthalenedicarboxylic acid]. The amount of the constituent units satisfies LNET≥0.5×mN.