PURPOSE: To improve the cross polarization protection ratio by inserting a first probe, which is perpendicularly arranged on a dielectric plate, toward the center of a round waveguide and inserting a second probe inward from the center part of one side of the section of a square waveguide.

CONSTITUTION: When a vertically polarized wave having an electric field parallel with the Y-axis direction and a horizontaly polarized wave having an electric field parallel with the X-axis direction are introduced into a round waveguide 2, the vertically polarized wave can be coupled to a probe 3 because the probe is attached in the direction parallel with the Y axis. Since the vertically polarized wave is reflected by a slot 6, it is not outputted to a square waveguide 7. the horizontally polarized wave is introduced into the square waveguide 7 through the slot 6 because the slot 6 is so arranged that its lengthwise direction is parallel with the Y axis, and the horizontally polarized wave is propagated in the square waveguide 7 with the electric field in the X-axis direction. Since a probe 8 is attached in the direction parallel with the X axis, the horizontally polarized wave can be coupled to the probe 8. The horizontally polarized wave and the vertically polarized wave are separated by the slot 6, and thus, the cross polarization protection ratio is improved.