PURPOSE: To enable the stable production of a prepreg which has excellent dielectric properties and is excellent in chemical resistance and heat resistance after cure by dissolving with heating a polyphenylene ether resin in a specific solvent in a given 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 at 50°C or higher in at least one solvent selecting from among benzene, toluene, xylene, and ethylbenzene in an amount of up to 12g 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 material, etc., in the electronic industry, etc.