A polarization coupler includes: connector waveguide that connects circular waveguide with quadrangular waveguide arranged in an axial direction of circular waveguide and having short side shorter than an inner diameter of circular waveguide; flat conductor wall formed over connector and circular waveguides, and dividing the inside of connector and circular waveguides arranged parallel to an extending direction of long side of quadrangular waveguide; first inclined surface formed on inner wall of connector waveguide at a position facing one surface of conductor wall, and inclined toward conductor wall as coming closer to quadrangular waveguide; second inclined surface formed on the inner wall of connector waveguide at a position facing the other surface of conductor wall, and inclined toward conductor wall as coming closer to quadrangular waveguide; and coupling hole, formed in circular waveguide, for extracting one polarization-divided by conductor wall out of electromagnetic waves propagated through circular waveguide.