PURPOSE: To obtain a coaxial waveguide converter from which harmonic waves are eliminated without complicating the system by forming integrally a harmonic wave eliminating circuit to part of a microstrip line.

CONSTITUTION: Part of a rectangular waveguide 1 is formed open, in which a coaxial line 2 is attached integrally. An antenna 3 is provided in the inside of the waveguide 1 while being projected to the line 2. A microstrip line 6 as the conductive film is formed to the surface of a dielectric base 4 in the antenna 3 and its line impedance is selected to be matched with that of the waveguide 1. Part of the pattern at the tip of the line 5 is formed differently to be a harmonic wave eliminating circuit 6 comprising the microstrip line. Thus, even if a harmonic wave is included in an output from an amplifier connected to the line 2 in this constitution, the harmonic wave is eliminated by the circuit 6 by combining the circuit 6 with the HPF characteristic of the waveguide 1 and not outputted through the waveguide.