An apparatus for forming images on at least one surface of a sheet includes a reader for reading image information on documents, a memory for storing image signals output by the reader, and an image forming unit for selectively forming one of: (i) two page toner images corresponding to the image signals of two pages of the documents separately on two image carrying members, and (ii) a single toner image on one of the two image carrying members. A jam detector detects a jammed sheet, and a delivery detector detects delivery of a copied sheet to outside the apparatus. Image signals are eliminated from memory in accordance with a progression of image formation. If the jam detector detects the occurrence of the jammed sheet, a control unit prohibits the memory from eliminating the image signals until the delivery detector detects a next sheet delivered from the apparatus, so that the selected image expected to be formed by the image forming unit on the jammed sheet may be properly formed on a following sheet based on the image signals retained in the memory. A selector is provided for selecting one of a both-surface copy mode and a single-surface copy mode, wherein the control unit controls the memory to store the image signals corresponding to two pages of the documents when the both-surface copy mode is selected and to store the image signals corresponding to the single page of the documents when the single-surface copy mode is selected.