A coaxial microstrip line conversion circuit includes: a waveguide including first and second through holes, spaced apart from each other, the second through hole having a dimension to cut off a used frequency; a coaxial connector including a central conductor including a projection projecting from an axial end of an outer conductor; and a microstrip line including a ground conductor provided on one surface of an insulating substrate, and a strip line provided on the other surface of the insulating substrate and including a projection projecting axially from the ground conductor. The outer conductor is connected to an outer wall of the waveguide. The projection of the central conductor is inserted through the first through hole into the waveguide, the ground conductor is connected to an inner wall of the second through hole, and the projection of the strip line is inserted through the second through hole into the waveguide.