The present invention provides an illumination optical system which illuminates a surface to be illuminated with light from a light source, the illumination optical system including a plurality of illumination systems configured to form predetermined illumination regions with the light from the light source, an optical system having reflecting surfaces, which are configured to reflect the light beams from the illumination systems, respectively for the plurality of illumination systems, and a light-shielding unit configured to shield a certain light component in a composite illumination region formed by the light from the optical system to shape a shape of the composite illumination region, wherein one continuous composite illumination region is formed on the surface to be illuminated by connecting the respective illumination regions by reflecting the light beams from the plurality of illumination systems by the reflecting surfaces.