To provide a compact and inexpensive antenna device of a simple constitution by setting the space between the slots, cut on a waveguide wall at the value equal to the inter-element horn space that cancels the side lobe as the feeding points of the element horns and then supplying the desired electric power to secure a desired antenna beam.

If the internal wavelength of a slot waveguide 1 is set referred to as λg, and the standing waves are generated in the waveguide 1 in every λg/2. Thus, an almost even field level is secured among the slots, as long as these slots are cut in every λg/2. Thereby, the slot spaces 20 and 21 are set at λg/2. A space 22 is set at λg/4 between the end of the waveguide 1 and a slot 7, since this end is used as a short-circuit plate. An input radio wave 11 has the components of electric fields 23, 24 and 25 on the slots 5, 6 and 7 respectively. The electric field levels are decided according to the deviation value to or every slot from a center line 18. Thus, the position of the slot 5 is decided, so as to secure a desired electric field.