Connection structure of waveguide, waveguide connector, mode converter, and waveguide unit

A connection structure of a waveguide includes a rod-like dielectric, and an outer conductor. A three-dimensional component includes a connection surface, an insertion hole, and a corner which forms an opening edge of the insertion hole over an entire circumference on the connection surface, the connection surface at least partially including a conductive region to which a connection enlarged portion of the outer conductor is connected, the insertion hole having conductivity over an entire circumference of an inner surface, the corner having conductivity and being conducted with the inner surface of the insertion hole. In a state where the waveguide and the three-dimensional component are connected with each other, the connection enlarged portion is electrically conducted with the inner surface of the insertion hole through electrical connection with the connection surface and the corner, and smooth connection is made at the corner.