To provide a means detecting a displacement in the case of an installation and the case of an ADF opening and closing and correcting the quantity of the oblique movement of an original in the case of an oblique reading resulting from the displacement.

1. A carrying roller in the vicinity of the place of the oblique reading is read by a CCD under an original-free state. 2. Vertical-scanning places, where brightness levels reach a maximum value, are detected respectively at two places mutually separated at a proper interval L in the horizontal scanning direction in a read image. 3. The difference of the detected vertical-scanning positions at two places is computed, and the quantity of a correction is determined.