In an image forming apparatus having a two-face copy mode wherein document images are formed on front and back faces of a paper, a direction of an image such as portrait or landscape in a document is set by a user or according to a document size, while a first margin is also set by a size or a document size for a document image on the front face in a reproduced image on a paper. A second margin for another document image on the back face is determined at a position opposite to the first margin. An image memory stores the digital image data. A rotation angle is set according to the document direction and the margin position, so that normal images can be observed in the front and back faces of a paper if the paper is turned over by fixing it at the margin. Then, the digital image data read from said image memory is rotated by the rotation angle. Then, the digital image data is printed on the front and back faces of a paper. If a 2-in-1 copy mode is provided, the rotation angle is determined similarly except the setting or document direction which is set opposite to that of documents for a synthesized image of two document images.