A polygonal mirror unit is adapted to prevent contamination using a photocatalyst, a light scanning unit, and an image forming apparatus employing the polygonal mirror. The polygonal mirror unit is designed to prevent contamination of organic materials on a reflective surface. The polygonal mirror unit includes a driving device and a polygonal mirror having a reflective surface that is rotated by the driving device and deflects and scans an incident beam. The polygonal mirror has a main body with a plurality of reflective surfaces and photocatalytic layers formed on the plurality of reflective surfaces. The photocatalytic layers are activated by an incident beam of a predetermined wavelength to decompose organic materials on the reflective surfaces.