A digital copier includes an image reading part and a printing part. The image reading part includes reading means for electrically scanning lines of image information of a document in a primary scanning direction at a predetermined cycle to output image signals for the lines; secondary scanning means for mechanically moving a primary scanning position with respect to the document at a constant secondary scanning speed V in a secondary scanning direction perpendicular to the primary scanning direction; and line thinning means for thinning out lines for the image signals to 1/N where N is an integer. The printing part prints an image on a printing medium based on the image signals supplied from the image reading part. The lines of image information of the document are copied at an arbitrary zoom ratio in the secondary scanning direction by combining a value of the integer N and a value of the secondary scanning speed V.