The present invention provides a waveguide slot antenna which utilizes a waveguide as a feeding line and has a linear-shaped slot provided in a wall of the waveguide. The waveguide slot antenna is characterized in that it comprises a pair of polarized wave conversion members surrounding an outer periphery of the slot and divided by a slit intersecting the slot. The present invention can provide a waveguide slot antenna capable of radiating a circularly polarized wave with a satisfactory axial ratio characteristic, over a wide band, only by adding a simple component to a conventional waveguide slot antenna.