An image processing unit (20) includes a averaging circuit (21) and a memory (22) for storing pixel data. The memory (22) is a memory for storing the pixel data subjected to analog-to-digital conversion. The averaging circuit (21) performs an averaging process between two adjoining pixels on each line stored in the memory (22). The digital pixel data is stored in the memory (22) and the averaging process is performed between the two pixels in the main scanning direction using the pixel data read by a first photoelectric conversion element trains and the pixel data read by a second photoelectric conversion element trains. Therefore, the noise component can be reduced, whereas the S/N ratio can be improved. Accordingly, image quality is made improvable with a simple arrangement.