After a trench 54 is formed in a conductive foil 60, a circuit element is mounted in a flip chip method. Then, an insulating resin 50 is covered on the conductive foil 60 as a support substrate. After reversion, the conductive foil 60 is polished over the insulating resin 50 as a support substrate at this time to separate the conductive paths. Accordingly, a circuit device having the conductive paths 51 and the circuit elements 52 supported by the insulating resin 50 can be produced without employing the support substrate.