PURPOSE: To suppress secondary electron discharge and to prevent the temperature of a high-frequency transmitting window from being increased by forming a solid solution film composed of titanium nitride and titanium oxide on the surface of the transmitting window composed of a dielectric ceramic board to be formed in a waveguide.

CONSTITUTION: Between a pair of rectangular waveguide parts 15 and 15, a circular waveguide part 11 is crimped through circular flanges 14 and 14. At the circular waveguide part 11, a transmitting window 12 composed of the dielectric ceramic board such as alumina, etc., is provided. On the both sides of the transmitting window 12, solid solution films 13 and 13 with high resistance composed of titanium nitride and titanium oxide are formed by sputtering in the discharged gas of Ar, N and O2 with the thickness of 60-130. The transmitting window 12 is cooled by a water duct 16. The left side of the figure is connected to the vacuum side of klystron and the right side is connected to an external waveguide side. Thus, the discharge of secondary electrons is suppressed, the temperature of the transmitting window is prevented from being increased, and the crack or melting of the transmitting window is prevented.