A heat insulating transmission line includes a first waveguide with a first aperture end, a second waveguide with a second aperture end, and a reflector. The second waveguide is arranged coaxially with the first waveguide. The second aperture end faces the first aperture end through an air gap. The reflector is provided outside the air gap, and controls radiation power from the air gap. In addition, the reflector is substantially parallel to a portion of a virtual plane connecting an inner wall of the first aperture end of the first waveguide and an inner wall of the second aperture end of the second waveguide. When a mean frequency of a signal transmitting through the heat insulating transmission line is expressed as λ, a distance between the virtual surface and the reflector is not less than N×λ/2−0.05λ and not more than N×λ/2+0.2λ (N is a positive integer).