PURPOSE: To accurately measure the position of side and independently of disorder of perforation at the side end, by providing a means which decides by majority of measurements of ≥3 times of scannings of a detector and determining the side end position of a document with the decision by majority.

CONSTITUTION: A scanning section 20 scans a document from one side end to another side end as the main scanning direction and scans for one line's share toward the sub-scanning direction, converts data into a binary information and stores it in a picture memory 40 via a gate 30. A CPU50 closes the gate 30 and stops a document 10 via a document run control section 60. A photodetector of the section 20 corresponds to a photodetector in which the main scanning direction of document run plane, e.g., 0.105mm corresponds to one photodetector. The sub-scanning is made in the pitch of 0.105mm. Thus, the picture is sectioned into 0.105mm and each section is stored to each one bit of a picture memory 40. The measurement of the document side ends is made with the picture memory 40 and the CPU50 after the said storage is finished.