PURPOSE: To markedly improve the melt flow of a thermoplastic resin by using, as a modifier, a carbonate-type compound of a molecular weight in a specified range obtained by reacting a specified dihydric phenol with a carbonate precursor by using a monohydric phenol as a terminating agent.

CONSTITUTION: A carbonate precursor (e.g. phosgene) is introduced into a dihydric phenol represented by formula I (wherein R1 to R4 are each H, halogen, a monovalent aliphatic or aromatic group, and W is a single bond, alkylene or the like) (e.g. bisphenol A) in the presence of an acid binder (e.g. sodium hydroxide) and a solvent (e.g. methylene chloride). To perform this reaction, a monohydric phenol represented by formula II (wherein R5 is a bivalent aliphatic group; R6 is a monovalent aliphatic group or halogen; K is H, a monovalent aliphatic group or the like; p is 0-4; (m) is 4-20; and (n) is 1-100) is used as the terminating agent, and a tert. amine or the like is added as the catalyst. The pH in the reaction is kept at usually 10 or above, and the reaction is performed at 0-40°C for several min to 5hr.