A functional elemental device and an FET sensor provided with the same, formed by forming a conductive carbide layer between the substrate and a conductive carbon material layer, and organic thin films and were formed on the surface of said conductive carbon material layer.Thereby, the adhesiveness between the substrate and the conductive carbon material layer can be remarkably improved, and peeling does not occur in the case of electrolytic polymerization reaction and film formation, and since the interstitial ions and the like from the substrate can be prevented, the lowering of the function can be prevented, and therefore, there is attained such an effect that the preparation of a thin film covered electrode becomes possible.