PURPOSE: To obtain titled composition without impairing electrical insulating performance, by incorporating unsaturated polyester resin composition containing inorganic filler with, as part of said filler, silane coupling agent-treated inorganic hollow particles.

CONSTITUTION: The objective composition comprising (A) 100pts.wt. of a composition made up of (i) unsaturated polyester resin, (ii) inorganic filler (e.g. calcium carbonate), and (iii) inorganic hollow particles treated with ≥0.1pt.wt. of silane coupling agent (e.g. aminosilane, vinylsilane), (B) polymerizable monomer (e.g. styrene), (C) curing catalyst (e.g. benzoyl peroxide), (D) shrinkage-reducing agent (e.g. polystyrene) and (E) releasing agent. Said hollow particles contribute to significant reduction in the specific gravity of the final composition, whereas said silane coupling treatment serving for retaining the electrical insulating performance of the same.