PURPOSE: To obtain the titled composition having excellent thickening viscosity after reaching molding adaptability viscosity range, extremely reducing bad molding such as short shot or reduction in appearance, containing a specific unsaturated polyester, a polymerizable monomer and a thickening agent.

CONSTITUTION: The aimed composition containing (A) an unsaturated polyester modified dicyclopentadiene in the presence of an organic phosphorous ester (triphenyl phosphite), (B) a polymerizable monomer (e.g. styrene or divinylbenzene) and (C) an organic thickening agent (preferably 4,4'- diphenylmethane diisocyanate or hexamethylene diisocyanate) and/or an inorganic thickening agent (e.g. magnesium oxide or calcium oxide).