An optical apparatus for irradiating light emitted from a light source onto a predetermined object, comprising a polarized light separating optical element for passing a first of two linearly polarized light beams polarized in directions perpendicular to each other and reflecting a second of said two linearly polarized light beams of light emitted from said light source; a phase shifter for converting said first linearly polarized light beams passed from said polarized light separating optical element into substantially circularly polarized light; and a reflecting mirror for reflecting said substantially circularly polarized light passed from said phase shifter back toward and through said phase shifter so that said circulatory polarized light is converted into a third linearly polarized light beam, wherein light emitted from said polarized light separating element, passed through said phase shifter, reflected by said reflecting mirror, and passed back through said phase shifter is irradiated on said predetermined object.