PURPOSE: To set freely the relation of position of an input/output section by connecting a coaxial waveguide converting waveguide to a coaxial waveguide part by a coaxial connection part so as to constitute a waveguide interface.

CONSTITUTION: A coaxial connector 17 for interfacing with an external circuit is connected to one end of a metallic chassis 19, one end of a hollow metallic outer conductor 23 is connected to the bottom face near the right end of the metallic chassis 19 and the waveguide 21 for coaxial waveguide conversion whose one end is opened is connected to the other end while the coaxial center is made in parallel with that of the coaxial waveguide comprising the metallic chassis 19 and a metallic cover 20. Then the waveguide interface is formed by fitting the waveguide for the coaxial waveguide conversion to the coaxial waveguide by the coaxial connection part. Thus, it is possible to select freely the relation of the center position of the coaxial connector 17 and that of the waveguide 21 by changing the length L of the metallic outer conductor 23.