A light scanning unit having a simple structure and a low-priced multi-beam shape is provided. The light scanning unit includes a composite light source in which a plurality of light sources for emitting coherent light having different wavelengths are arranged adjacent to one another and an optical axis of each light source is arranged substantially parallel to each other and which emits light at a divergence angle centering on each optical axis. The plurality of coherent emitted light comprising a light beam. The light scanning unit further comprises a collimator lens which is arranged on an approximately central axis of the optical axis of each light source and collimates the light beam emitted from the composite light source, and a cylinder lens for condensing the light beam emitted from the collimator lens, a polygonal rotating mirror for scanning the light beam condensed by the cylinder lens in a main scanning direction on an exposed surface of an exposed object. The light scanning unit also comprises an optical system comprising one mirror or a group of mirrors, which condenses light irradiated by the polygonal rotating mirror onto an image surface and makes a scanning speed of an exposed point be nearly uniform.