PURPOSE: To omit flatness alignment conducted with the use of a measuring instrument at mounting by devising that a face of a flange being a reference face is pressed into contact onto a block.

CONSTITUTION: A flange clipping tool comprising a retaining metallic fixture 2, a bolt 3 and a block 4 is fitted reversely in comparison with that of a conventional method. The block 4 is provided with a step 15 to which a corner of a flange 6 is fitted. As the mount method, a face A of the flange 6 processed already is pressed into contact onto the step 15 of the block 4 and also onto the side face of the flange 6 at the same time to position the retaining position. Then the entirely same mounting method as the conventional method is used after the waveguide with flange is fixed by a couple of flange clipping tools. Thus, the flatness alignment at mounting is omitted.