An optical system includes semiconductor lasers arranged in the direction of the slow axis of the laser beam, an optical means which makes parallel the collimated laser beams, an optical member which is provided with inlet and outlet faces which are positioned in perpendicular to the optical axis of laser beams and total reflection surfaces which are opposed to each other at a space where the component in the direction of the slow axis of the laser beam entering from the light inlet face repeats internal reflection, and emits from the light outlet face a slow axis uniform laser beam, and an imaging optical means which images the slow axis uniform laser beam on a surface as a linear line beam extending in the direction of the slow axis.