PURPOSE: To surely shut-off a read optical system at a prescribed position with high accuracy by using a line sensor so as to sense sensing marks on a positioning member successively at a region in precedence over a return scanning end scheduled position.

CONSTITUTION: A positioning member 3 is disposed on the outside of a read area of an original and sensing marks 4, 5 are disposed on a position on the member preceding the read area. A line sensor 12 and a mark detection circuit 13 detect the sensing marks and a read starting position 4 and an end position of return scanning are decided based on the relative position. Thus, the return scanning end position is decided surely with high accuracy.