PURPOSE: To obtain the titled low-melting resin useful as a surgical gypsum material, by subjecting -caprolactone or hydroxycaproic acid, a specified hydroxy component and a specified acid component to an esterification reaction and a polycondensation reaction.

CONSTITUTION: γ-Caprolactone or hydroxycaproic acid and a hydroxy component comprising at least one 2W18C aliphatic diol are mixed with an acid component selected from the group consisting of a 4W12C or 36C aliphatic dicarboxylic acid, and its anhydride or lower alkyl ester, and the resulting mixture is subjected to an esterification reaction and a polycondensation reaction in the presence of an esterification-promoting catalyst. Thus, there is obtained a thermoplastic polyester resin, number-average MW 5,000W200,000, having a compositional formula[A]l[B]n[C]n, wherein A is a 4W12C or 36C aliphatic dicarboxylic acid residue, B is a 2W18 C aliphatic diol residue, C is a 6-hydroxycaproic acid residue, these residues are bonded to each other through ester linkages and distributed over the entire molecule at random and/or as blocks, l, m and n are each greater than 0, and the 6-hydroxycaproic acid residue is present in an amount of 60W98wt%.