PURPOSE: To obtain the waveguide slot array antenna which has small loss and a wide frequency band and is easily reduced in size by constituting a rectangular waveguide which has a slot and a diopole or monopole as one-wavelength periodic structure as an aperture for radiation on a flank and radiating a circular polarized wave in the front direction of an aperture surface.

CONSTITUTION: A rectangular waveguide 2 for radiation with in-tube wavelength λg is arranged having its long sizes horizontally and is fed electrically from a power feeding part 1. Further, a standing wave distribution is generated in the waveguide by a short-circuit terminal 4. Slots 11, 11,... are arrayed in the waveguide at λg, intervals as apertures for power radiation of a (z)-polarized wave while one-sided in a (+x) and a (-x) direction in parallel to a (z) axis, and also shifted by λg/2 in the (z) direction on the (+x) and (-x) sides. Further, dipoles 12, 12... composed of U-shaped apertures 14, 14... as the apertures for the power radiation of the (x)-polarized wave are arrayed in the waveguide in its center at the same time at intervals λg in the (z) direction parallel to the (z) axis.