PURPOSE: To facilitate the production of a prepreg which has excellent dielectric properties and is excellent in chemical resistance and heat resistance after cure by dissolving a polyphenylene ether resin in tetrahydrofuran in a specific concentration and infiltrating the solution into a substrate.

CONSTITUTION: A resin composition comprising a polyphenylene ether resin such as poly(2,6-dimethyl-1,4-phenylene ether), a styrene graft polymer thereof, or a 2,6-dimethylphenol/2,3,6-trimethylphenol copolymer is dissolved in tetrahydrofuran in an amount of up to 20g per 100ml of the solvent. The solution is infiltrated into a substrate at room temp. to obtain the prepreg, which has a satisfactory surface state and is suitable for use as a dielectric, insulating, or heat-resistant material in the aerospace industry, electronic industry, etc.