PURPOSE: The titled composition, prepared by incorporating an unsaturated polyester resin having maleic anhydride with a thermoplastic resin and a specific curing acceleration assistant respectively in specific proportions, and having stable low shrinkage property even in molding at a low temperature with a small reduction in strength.

CONSTITUTION: A low shrinkable resin composition prepared by incorporating (A) 100pts.wt. unsaturated polyester resin consisting of (i) 50W70mol%, based on the total acidic component, maleic anhydride or fumaric acid and (ii) a polymerizable monomer, e.g. styrene, with (B) 10W50pts.wt. thermoplastic resin, preferably polystyrene, and (C) 0.01W0.2pts.wt. amine compound, preferably dimethylaniline, or a keto-enol tautomeric compound, preferably acetylketone, as a curing acceleration assistant.