In a signal processing apparatus for transmitting or processing a signal, a substrate has a recessed portion in a surface of the substrate, a first interconnecting conductor is on the substrate, including at least the recessed portion of the substrate, and a dielectric support film is on the substrate opposite the recessed portion of the substrate with an air space between the dielectric support film and the substrate. A second interconnecting conductor is on a part of a surface of the dielectric support film. The apparatus has a simple structure and reduced transmission loss and can be made in a simple manufacturing process.