PURPOSE: To prevent a reading ratio from being lowered by displaying picture deterioration caused by the dirt of an optical unit on a display device.

CONSTITUTION: Since a postal matter 12 moves, a beam reflected from a part irradiated with light is passed through an optical window 16 and detected by a CCD 18 as a photodetection part, and a photoelectric converting signal is transmitted to a picture recognition part. A projecting pattern is prepared from a picture signal coming from a photoelectric conversion part, and it is judged whether there is a maximum projecting value Y or not. Only when a black line 22 is detected at the same place throughout 10 postal matters, it is judged as dirt, a message is displayed on a display panel, and it is informed of an operator that dirt is attached. Thus, the postal matter is prevented from being unreadable by the dirt of the optical window, and therefore, the reading ratio of the postal matter is prevented from being lowered.