To obtain a proper both-surface image even on different conditions by executing the mirror-image rotation of an original image on a stage that the toner image of a back surface is formed when the effective image area of the original image and that of a transfer material are in identical directional relation.

When the effective image area of the original image is in the identical directional relation with respect to that of the transfer material at the back-surface image forming time, at the time of forming the toner image on the transfer material, the mirror-image rotation of the original image is executed on the stage that the toner image of the back surface is formed in a data selector (I) B17. The image data of the front surface accessed from a memory B14 is processed by an image processing part (II) B16 and rotated by 90° by the selector (I) B17. Thereafter, it is inputted to a memory for outputting front-surface image data B18A. Next, the mirror-image rotation processing or the (mirror-image + 90°) rotation processing of the image data of the back surface is executed according to the condition. Thereafter, it is inputted to a memory for outputting back-surface image data B18B. Then, the image data of the back surface is outputted to an exposure unit 12 and the back-surface image is formed on a photoreceptor drum 10. Next, the front-surface image is formed.