PURPOSE: To reduce a slit width and to decrease the leakage of an electromagnetic wave by sandwiching a thin metallic plate with a thin insulator and inserting it into a slit provided on the H surface center of a rectangular waveguide and directed toward a shaft direction.

CONSTITUTION: A metallic plate inner conductor 10 is sandwiched with two insulating sheets 11, a sheet 11 is sandwiched and supported by a stand 8 composed of two blocks. The conductor 10 and the sheet 11 are inserted through a slit 2 into a rectangular waveguide 1, and moved in the shaft direction together with the stand 8 along a slit 2. The electromagnetic wave added to an input edge 3 is introduced from the slit 2 through a coaxial line converting connector 9 to the waveguide 1 by the conductor 10. The conductor 10 is protected from the shorting with the slit 2 by the sheet 11 and can be moved. Thus, since the clearance of the side surface of the sheet 11 and the slit 2 can be made smaller, the slit width can be reduced.