A flowable insulating resin 1 is placed in a sheet-like molding die space and conductive magnetic particles 2 are dispersed in the insulating resin 1. A first magnetic field G1 is acted in the sheet thickness direction on the position where a conductive path is to be formed in the die space, and conductive magnetic particles 2a are locally collected together to form a conductive path. Simultaneously, a second magnetic field G2 is acted in the sheet thickness direction on the intermediate region and the magnetic field G2 is moved in the lateral direction to move conductive magnetic particles 2b left in the intermediate region to join the collection forming the conductive path. As a result, the number of the conductive magnetic particles left in the insulating resin becomes smaller.