Provided is a microwave plasma discharge lamp apparatus which includes a rectangular waveguide having a rectangular shape one end of which is closed and the other end is open and receiving a microwave through an opening to put out linearly polarized microwaves; a discharge lamp; a resonator cavity, formed in a cylindrical shape, one end of which is open, which is disposed to surround the discharge lamp, and which is made of a conductive mesh, thereby allowing the passage of the light from the discharge lamp; and a phase shifter, which has a cross-shaped waveguide opened in a propagation direction of the linearly polarized microwaves, is disposed between the other end of the rectangular waveguide and one end of the resonator cavity, and receives the linearly polarized microwaves from the rectangular waveguide to generate elliptically polarized microwaves in the cylindrical resonator cavity. The elliptically polarized microwaves discharge the discharge lamp.