To obtain a waveguide assembly which can easily fix a dielectric plate with a reduced number of parts, without having to use a fixing adhesive and, which is inexpensive and high in reliability.

Two opposing insertion guide grooves 3a and 3b are provided on an inner wall surface 100a of the waveguide 100 along the longitudinal direction from one end opening 7 of the waveguide. The dielectric plate 20 is inserted into the waveguide 100 along the guide grooves 3a and 3b from one end opening 7 of the waveguide 100. Through-holes 5a and 5b, passed through the bottom of the grooves 3a and 3b from a predetermined position of the outer wall of the waveguide 100, are provided as fixing means, and projections 6a and 6b having resiliency formed at a position opposed to the through-holes 5a and 5b of the plate 20 and projected into the holes 5a and 5b are provided. When the projections 6a, 6b are fitted into the through-holes 5a, 5b, the plate 20 can be easily fixed to inside of the waveguide 100, using a small number of parts.