PURPOSE: To establish microwave transmission window very stable thermally and electrically, by effectively removing the heat dissipation of dielectric substance, through flowing coolant between the metal tube and support supporting dielectric substance.

CONSTITUTION: The coolant 6 is flowed from the nipple 5 of water cool type between the metal tube 3 and the support 4 supporting the dielectric substance 2 and flowed from other water cooling nipple 5. Further, in transmitting microwaves, the waves propagates to other waveguide 1 through the waveguide 1, metal tube 3, and dielectric substance 2. In this case, heat will be caused by the dielectric loss of the dielectric substance 2, but since the heat will be carried out externally by the coolant 6, the temperature rise can completely be prevented.