A waveguide orthomode transducer includes a waveguide including a first waveguide portion and a second waveguide portion placed along a transmission direction of radio signals, the size of an aperture of the second waveguide portion smaller than the size of an aperture of the first waveguide portion, a first probe disposed at a first position, a second probe disposed at a second position, a third probe disposed at a third position, and a fourth probe disposed at a fourth position, wherein at least two of the first position, the second position, the third position, and the fourth position are located in the same plane perpendicular to the transmission direction of the radio signals.