PURPOSE: To prepare a copolymerized polyester having excellent transparency and color tone, in a short time, by adding a sulfonic acid compound to a catalyst in the copolymerization of ethylene terephthalate with a specific amount of ethylene oxide adduct of bisphenol A.

CONSTITUTION: The objective copolymerized polyester is prepared by adding 0.1W30mol% ethylene oxide adduct of bisphenol A of formula (m and n are 1W 50) to ethylene terephthalate, adding a sulfonic acid compound (e.g. aliphatic sulfonic acid) and/or sulfinic acid compound (e.g. aliphatic sulfinic acid) as a catalyst, and if necessary adding a metallic compound catalyst, a stabilizer, a color modifier, etc. to the mixture, and subjecting the composition to polycondensation reaction.

USE: Fiber for clothing and industrial use, etc., formed article such as film, bottle, etc.