This invention has an object of reading a high-quality image by detecting and correcting dust or scratches present on a transparent original in an image reading apparatus for reading, e.g., a transparent original. To achieve this object, an original illumination unit in the image reading apparatus for reading an original includes at least a first light source for emitting light in a visible region, a second light source for emitting light in a n invisible region, and a light guide plate which has these light sources arranged at end faces and has first and second light guide patterns for guiding light beams from the first and second light sources to the entire light-emitting surface.