A circuit device manufacturing method is provided, wherein contaminants attached to the top surfaces of conductive patterns 21 are removed using plasma to thereby improve the adhesion of conductive patterns 21 to a sealing resin 28. By selective etching of a conductive foil 10, separation grooves 11 are formed, thereby forming conductive patterns 21. A semiconductor element 22A and other circuit elements are mounted onto desired locations of conductive patterns 21 and electrically connected with conductive patterns 21. By irradiating plasma onto conductive foil 10 from above, contaminants attached to the surfaces of separation grooves 11 are removed.