Dielectric waveguide comprising dielectric body having an exterior coated with an electrically conductive film. Region in one side surface couples to another dielectric waveguide, and slot in bottom surface exposes the dielectric body in an L-shape in two adjacent side surfaces, except for the one side surface. Front surface of printed circuit board has a ground pattern opposed to the slot, which includes opening with outer shape greater than slot, and a back surface having a ground pattern surrounding a strip line disposed to cross through the slot. A distal end of the strip line and the front surface-side ground pattern are coupled together by a via hole. The opening is surrounded by a via hole group which couples the front and the back surface-side ground patterns together. The dielectric waveguide is disposed to allow the opening and the slot to be opposed to each other.