An etching treatment is applied to a metal plate in accordance with a predetermined circuit pattern to form a circuit. Subsequently, a sandblast treatment is applied to an entire surface including the circuit to remove a conductive reactive layer remaining at a metal-removed portion of the circuit. Accordingly, an insulating substrate as an underlying layer is exposed from the metal-removed portion of the circuit. The sandblast treatment is performed under a condition in which an Ni plating layer remains on the circuit at a stage at which the conductive reactive layer remaining at the metal-removed portion of the circuit is removed.