PURPOSE: To attain the conversion between a circularly polarized wave and a linearly polarized wave within a length of a short phase circuit by providing 1st and 2nd phase delay means with different components from each other as the phase circuit in combination.

CONSTITUTION: A 1st phase delay means is a metallic block 11 which is made flat and prolonged and fitted to two opposite circular arcs on the surface of the inside of a waveguide in the lengthwise direction of the waveguide 10. A 2nd phase delay means is a dielectric plate 12 fitted to the metallic block 11 so that both long sides of the dielectric plate 12 are butted to the metallic block 11 at tilt of 45° with respect to an excitation probe 13 in parallel with an electric field component Y in the vertical direction being one of two linearly polarized waves orthogonal to each other. The 1st phase delay means and the 2nd phase delay means 12 to delay the phase as the phase circuit are provided to use both the means in common thereby attaining the conversion between a circularly polarized wave and a linearly polarized wave within a length of the short phase circuit.