An synthetic-image forming apparatus including: an interface for inputting image information from a computer; a scanner for inputting original document image information; an operation portion for inputting region information indicating a region in which images are synthesized; an operation portion for selecting a printer base mode in which the image information from the interface is synthesized onto outside of the region and image information from the scanner is synthesized onto inside of the region or a scanner base mode in which the image information from the interface is synthesized onto inside of the region and the image information from the scanner is synthesized onto outside of the region; image forming portion for synthesizing the image information from the interface and the image information from the scanner in accordance with the mode selected by the operation portion and the region information supplied from the operation portion and forming an image on a selected sheet in accordance with the synthesized image information; and a control portion that inhibits an image forming operation of the image forming portion if image size included in the image information supplied from the interface and size of the selected sheet do not coincide with each other in the printer base mode and that permits the image forming portion to start the image forming operation if the image size of the image information supplied from the interface and the size of the selected sheet do not coincide with each other in the scanner base mode.