A cavity configured by electrically connecting an earth conductor formed on a multilayer dielectric substrate and on which a plurality of high frequency circuits are mounted, and a shield cover member. A waveguide aperture is formed on the earth conductor on which the high frequency circuits are mounted and is electrically coupled to the cavity, and an end-short-circuited dielectric waveguide formed in a direction of layer lamination of the multilayer dielectric substrate is connected to the waveguide aperture, and has a length approximately ¼ of an effective wavelength in the substrate of a signal wave. Spatial isolation between the high frequency circuits is ensured by an inexpensive and simple configuration using the single cavity.