A light scanning unit includes: a light source unit; a polygon mirror for deflecting and scanning light emitted from the light source unit, in a main scanning direction, and having a plurality of deflection surfaces and a plurality of edges at which adjacent deflection surfaces meet one another; an image forming optical system for condensing the deflected light; and a synchronization detection optical system for detecting a portion of light that is divided and reflected at an edge of the polygon mirror. The light scanning unit may be incorporated into an image forming apparatus.