PURPOSE: To obtain a prepreg improved in adhesion between the glass cloth base and the resin, by impregnating a glass cloth base impregnated with a coupling agent diluted with an organic solvent with a resin varnish and drying it.

CONSTITUTION: A glass cloth base (A) optionally surface-treated with an aqueous solution of a coupling agent is impregnated with a solution (B) of a coupling agent (e.g., γ-aminopropyl-triethoxysilane) diluted to 0.5W5% with an organic solvent (e.g., dimethylacetamide), impregnated with a resin varnish (C) (e.g., polyimide resin varnish) and dried.