PURPOSE: To reduce the conversion loss by using a ridge waveguide so as to convert an electromagnetic wave in the coaxial mode outputted from a circular waveguide into an electromagnetic wave in the waveguide mode and outputting the wave with respect to a horizontally and vertically polarized wave switching feed horn.

CONSTITUTION: A ridge waveguide 6 formed by providing a taper on a ridge structure section 8 at an opening is coupled to the outer wall of an end face 4 of a circular waveguide 2 while its axis is in a direction intersecting orthogonally with the prolonged line of a guide axis of a circular waveguide 2. A probe 3 having a turning axis perpendicular to the end face 4 is provided and inserted in the ridge waveguide 6 while being prolonged to the outside from the opening 5, and connected to an insulation shaft in the inside of a through-hole 12 provided to the ridge structure section 8 and penetrated through the ridge waveguide 6 and connected to a drive section 7 provided on the outside. Then the probe 3 is turned around the guide axis of the circular waveguide 2 as a center, one electromagnetic wave among plural electromagnetic waves introduced to the circular waveguide 2 is coupled, the electromagnetic wave is sent by the probe 3 in the coaxial mode to the ridge waveguide 6, and a signal is outputted from the ridge waveguide 6.