An electroconductive circuit, in which a masking material and a substrate both have low dielectric loss tangent to high-frequency signal and have excellent adhesion to each other, can be formed in a simple and low-cost manner. A cycloolefin resin with a flexible polymer mixed and dispersed therein is injection molded to form a primary substrate 1, and a cycloolefin resin having compatibility in which a flexible polymer has not been mixed, is injection molded on the surface of the primary substrate 1 to form a masking layer 2. Since the cycloolefin resin itself has etching resistance, for onlythe surface of the primary substrate 1 not covered with the masking layer 2, that is, only a part 1a on which an electroconductive circuit is to be formed, the flexible polymer can be dissolved for roughening and only the part 1a can be rendered hydrophilic. Accordingly, an electroconductive layer 4 can be selectively formed by electroless plating only onto the part 1a not covered with the masking layer 2.