To provide the installing structure of a window member for high frequency wave which can efficiently output high frequency electromagnetic waves, and can prevent the peeling from a waveguide and the generation of cracks and splits.

In the installing structure of a window member 2 for high frequency wave made by jointing the window member 2 for high frequency consisting of a silicon nitride, on the inner periphery of a waveguide 1 consisting of copper, a circular member 3 which consists of a material having a thermal expansion coefficient less than 7ppm/°C is joined on the outer periphery of the copper waveguide 1, at the position corresponding to the window member 2 for high frequency jointed on the inner periphery. Consequently, the generation of a thermal stress resulting from the difference of thermal expansion coefficients of the waveguide 1 and the window member 2 can be suppressed, and the peeling of the window member 2 from the waveguide 1, and the generation of cracks and splits can be prevented.