PURPOSE: To aviod the deformation of the waveguide part which is caused when a slot is processed, by preparing a pair of half-cut waveguides having slots and then butting them together.

CONSTITUTION: A pair of half-cut waveguide parts 10a and 10b are set previously with symmetrical faces which are parallel to a current distribution formed along the lengthwise direction. The slots 14 are previously formed to the part 10a, and then both parts 10a and 10b are butted and fixed to each other. In such a way, it is possible to avoid the deformation of the part 10a which is caused when the slot 14 is processed. At the same time, the thickness of the part 10a can be reduced as less as possible. As a result, the characteristics of a slot array antenna can be improved.