PURPOSE: To provide the titled composition giving light-weight BMC or SMC without lowering the electrical insulation, by compounding inorganic hollow particles washed with acetic acid.

CONSTITUTION: Inorganic hollow particles washed with acetic acid is added to an unsaturated polyester resin composition as a part of the inorganic filler. The concentration of the acetic acid to be used for washing is about 0.5W5N. For example, 100pts.wt. of hollow particles are mixed with ≥1pt.wt., practically 10W100pts.wt. of pure acetic acid, and stirred until the pH of the liquid layer becomes constant. The treated hollow particles are washed with water until the electrical conductivity of the washed waste water reaches ≤30μΩ/cm, when the particles are dried and used as the filler.