Provided is a dielectric waveguide having a good reflection characteristic also in a band on a low frequency side of a center frequency of a given operation band. A dielectric waveguide (1) includes: a waveguide region (12) which is defined by a first wide wall (21), a second wide wall (22), a first narrow wall (23), a second narrow wall (24), and a short wall (25) and which is filled with a dielectric; and a mode conversion section (31) which includes a columnar conductor (34) extending from a surface of the waveguide region (12) toward an inside of the waveguide region (12). A width (W2) of the short wall (25) is configured to be greater than a waveguide width (Wi) at a location (x=x1) at which the columnar conductor (34) is provided.