In an image forming apparatus capable of reading a book document while turning over pages of the book, image data representative of two spread pages of a book document are stored in a frame memory. Among the image data, image data matching the front of a sheet are read out of the frame memory first and then output on the front of a fresh sheet. Then, an image is formed on the rear of a sheet refed from a duplex copy tray or refeed tray. Alternatively, an arrangement may be made such that, among two pages of image data, the image data read last are out of the frame memory first and then output on the front of a fresh sheet, and then an image is formed on the rear of a sheet refed from the refeed tray.