PURPOSE: To provide a premix composition capable of giving a light-weight product without lowering the electrical characteristics, by compounding inorganic microballoons with specific organic fine powder.

CONSTITUTION: A composition having an insulation resistance of ≥1010Ω after boiling for 2hr and a specific gravity of ≤1.5, and obtained by compounding an unsaturated polyester resin, a crosslinking agent, a shrinkage-controlling agent, a filler and reinforcing fibers, and containing 2W5wt% inorganic microballoons and 0W10wt% organic fine powder incompatible with the unsaturated polyester resin as a part of the above filler. The unsaturated polyester resin is preferably the one obtained by proceeding the esterification reaction to an acid value of about 30W20. The organic fine powder is e.g. hardened epoxy resin, hardened phenolic resin, etc.