An image forming system includes a sheet accommodating section, an image forming section, a term management section, a detecting section, and a display section. The sheet accommodating section has a plurality of sheet trays each accommodating therein printing sheets with advertisement. The term management section has stored therein information on a term of validity of the advertisement as advertisement term information connected with a sheet tray. The detecting section detects whether or not the advertisement is currently valid, based on the advertisement term information. Also, the detecting section determines during printing whether or not the term of validity of the advertisement on the printing sheets accommodated in a sheet tray being used in printing has expired. The display section displays information that a sheet tray other than the sheet tray being used is present which accommodates therein printing sheets each carrying an advertisement which is currently valid, if any.