PURPOSE: To provide a process for forming a copolymer not having unreacted dicarboxylic acid contaminating a polymer present therein by interfatially reacting divalent phenol, a carbonate precursor and aliphatic dicarboxylic acid while adjusting the reaction quantity and pH.

CONSTITUTION: Dihydric phenol (A) such as 2,2-bis(4-hydroxyphenyl)propane, a carbonate precursor (B) such as phosgene and 2-20 mol.% with respect to the component A of 9-20C alphatic, α,ω-dicarboxylic acid (C) such as sebacic acid are interfacially reacted while pH is adjusted to 8-9 during a period corresponding to 70-95% of the introducing time of the component B and adjusted to 10-12 during the period of the residual time to obtain the objective copolyester carbonate.