A mode converter which converts between a mode of a post-wall waveguide and a mode of a line in which a strip-shaped conductor is formed on another substrate different from a substrate of the post-wall waveguide, is less likely to cause a transmission failure resulting from a change in environmental temperature. A mode converter (10) includes: a post-wall waveguide (PW) in which an opening (121) is provided in a wide wall (conductor layer 12); a dielectric substrate (15) having a main surface on which a strip-shaped conductor (16) is formed; and a joining member (solder 18) joining the wide wall (conductor layer 12) and the substrate (15), wherein in a plan view, the opening (121) and the strip-shaped conductor (16) overlap each other.