PURPOSE: The titled polyester that is derived from isophthalic dihalide and 2 different kinds of bisphenols, thus having specific 2 kinds of structural units and specific reduced viscosity, high heat resistance, and permitting industrially advantageous production.

CONSTITUTION: The objective polymer is obtained preferably by interfacial polycondensation reaction between a solution of isophthalic dihalide in an organic solvent and an aqueous alkali solution containing bisphenol-A and bis(4-hydroxyphenyl)-sulfone, thus being composed of the structural units of formula I (A is alkylene, alkylidene; R1WR8 are H, alkoxy) and formula II (R9WR16 are H, alkoxy) where the molar ratio of unit I :II is 95:5W5:95 and having a reduced viscosity ηsp/C of more than 0.4dl/g which is measured with a solution of 1g/ dl in a solvent mixture of 1:1 (in weight) phenol/tetrachlorethane at 30°C.