To prevent ACS from being erroneously decided due to coloring of black characters or the like, which is caused by jitter, by managing the position in the sub-scanning direction of a read image area.

An image processor is provided with a color discrimination means for discriminating between an achromatic area and a chromatic area of a document on the basis of image data obtained by reading the moving document and changes processing in following stages on the basis of the discrimination result in the color discrimination means, and the image processor manages the position in the sub-scanning direction of the read image by an effective image area signal START-X and a line synchronizing signal LSYNC and switches the discrimination operation of an ACS decision part 200 in accordance with the position in the sub-scanning direction.