A light source unit includes a light source group wherein light sources are arranged into rows and columns so as to form a planar configuration and a primary reflecting mirror group disposed on an optical axis of the light source group to reflect light beams emitted from the light source group as light beams whose sectional areas are reduced in a column direction by narrowing intervals between light beams emitted from the light sources making up the rows of the light source group, the primary reflecting mirror group includes different strip-like reflecting mirrors disposed on optical axes of light beams emitted from the rows of the light source group in a step-like fashion, and the reflecting mirrors are disposed to eliminate intervals between reflected light beams from the reflecting mirrors, thereby making it possible to reduce the sectional areas of the light beams.