PURPOSE: To provide a circularly polarized waveguide-microstrip line converter which can improve its cross polarization discrimination and noise figure.

CONSTITUTION: Four probes 3A to 3D are contained in a waveguide 1 where a circularly polarized wave is transmitted at the positions which are orthogonal to each other on the same plane. Then every two probes adjacent to each other are defined as a 1st pair of probes 3A and 3B and a 2nd pair of probes 3C and 3D respectively, and the phase shifters 5A and 5B are connected to the probes which have different tube axial directions between both pairs of probes. Thus the 1st pair of probes can form a converter for the circularly polarized wave of a desired circulation direction, and a 2nd pair of probes can form a converter for the circularly polarized wave of the opposite circulation direction respectively. Furthermore, the output of the 2nd converter is terminated by a terminating resistor 8 to absorb the undesired components of the opposite circulation direction. As a result, the cross polarization discrimination is improved even when the T transformers 6A and 6B are used to a synthesizing circuit.