In a dielectric waveguide-microstrip transition structure for mounting a dielectric waveguide on a printed-wiring board, one object of the present invention is directed to providing a further downsized structure as compared with a conventional structure, while maintaining an influence of displacement between the dielectric waveguide and the microstrip at a low level by means of non-contact coupling. The dielectric waveguide-microstrip transition structure has a dielectric waveguide containing a dielectric block and a conductor film covering an entire surface of the dielectric block, except a signal input/output portion, wherein a slot is formed in a bottom surface of the dielectric waveguide to expose the dielectric; a microstrip having an end which is openly terminated and disposed with opposing to and spaced apart from the slot of the dielectric waveguide; and a cavity containing a conductive wall surrounding the end of the microstrip and the slot of the dielectric waveguide, except a part of the microstrip being led out to connect to an external circuit.