An image reading unit which is capable of optimally reading images from both reflective originals and transparent originals. A first rod lens array 15 collects reflected light from a reflective original that reflects light. A first photoelectric conversion element array 14 reads an image of the reflective original based on the reflected light collected by the first rod lens array 15. A second rod lens array 202 collects light transmitted through a transparent original that transmits light. A second photoelectric conversion element array 201 reads an image of the transparent original based on the transmitted light collected by the second rod lens array 202. The first photoelectric conversion element array 14 is mounted on a first substrate 16. The second photoelectric conversion element array 201 is mounted on a second substrate 203. The first substrate 16 and the second substrate 203 are disposed separately, and the first photoelectric conversion element array 14 and the second photoelectric conversion element array 201 are disposed on different planes.