To reduce a grating lobe of a waveguide array antenna system.

Placing many slot antenna elements in a waveguide forms a transmission or reception waveguide array antenna system. Radio wave reflectors 103 two of which have each antenna element 102 inbetween in an arrangement direction of the antenna elements 102 are placed on a side face 104 of the waveguide 101. A height of the radio wave reflectors 103 from a radiation face 102a is selected to be 1/4 of an emitted radio wave wavelength λ0 or over. The radio wave reflectors 103 shield a radio wave emitted from each antenna element 102 in a direction of a grating lobe. Thus, the radio waves are not strengthened in the direction of the grating lobe, resulting in the reduced grating lobe as the entire waveguide array antenna system.