The image forming apparatus receives image data based on multiple document images of various sizes and/or orientations. The apparatus identifies areas that are not yet assigned with document images on a single sheet of paper and identifies whether any document image can be laid out in an identified area. In another aspect of the apparatus, a maximum document image size is detected from multiple document images, and a scale factor is calculated to cause the detected maximum size match with the size of the entire area of the paper or an area obtained by dividing the paper into equal parts. In another aspect of the apparatus, a scaling factor is calculated for each document image to cause the size of the particular document match with the size of an area obtained by dividing the paper into equal parts.