1. An image forming apparatus to form an image on a sheet by permanent color toner and decolorizing toner to be decolorized, comprising: a detector unit to detect whether a predetermined identification image for indicating execution of an image forming process by decolorizing toner is formed or not at least on either of a first side and a second side of a sheet which is a subject of the image forming process to be executed; and a process controlling unit to prohibit the image forming process by the permanent color toner on the sheet side on which the predetermined identification image is detected when the identification image is detected by the detector unit.