To provide high molecular weight poly(ethylene isophthalate/ethylene terephthalate) type copolyesters suitable for gas barrier containers and the like, excellent in heat resistance, impact resistance and transparency and reduced in the amount of cyclic oligomers generated.

Poly(ethylene isophthalate/ethylene terephthalate) type copolyesters has, as the acid component, isophthalic acid and terephthalic acid and, as the diol component, ethylene glycol and cyclohexanedimethanol, and a solution haze of not more than 15% and a glass transition point of not lower than 60°C. The molar ratio of the isophthalic acid to the terephthalic acid is 100/0 to 15/85 and the molar ratio of the ethylene glycol to the cyclohexanedimethanol is 95/5 to 30/70, and 1×10-4 to 200×10-4, based on one mole of the acid component, phosphorus compound component is copolymerized, and the number average molecular weight of the copolyesters is not smaller than 10,000.