PURPOSE: To obtain a polyester copolymer having a low melting point and a proper crystallinity by the copolymerization of a dicarboxylic acid or its ester- forming derivative, a low-molecular weight diol, and a high-molecular weight diol.

CONSTITUTION: A dicarboxylic acid or its ester-forming derivative (A) composed of terephthalic acid or a mixture of 50mol% or more terephthalic acid and one or more aromatic and aliphatic dicarboxylic acids other than terephthalic acid, a low-molecular weight diol (B) composed of a mixture of 1W50mol% 2-methyl- 1,3-propanediol and 99W50mol% 1,6-hexanediol, and one or more high-molecular weight diol (C) of polyalkylene ether glycols and polycaprolactone glycols (in such as amount that the proportion of the high-molecular weight diol component in the copolymer is 0W60wt%) are subjected to copolymerization.