In a color laser printer, four toner cartridges are juxtaposed in a straight line in a process unit. When the process unit is withdrawn from the laser printer to a position at which the toner cartridge nearest the front of the laser printer can be removed, an out-of-toner sensor farthest from the front of the laser printer from among four out-of-toner sensors turns on. Side surfaces of a unit-accommodating section constituting the process unit block detection light from the other three out-of-toner sensors so that these out-of-toner sensors remain off. Since the farthest out-of-toner sensor switches from off to on, the CPU determines that the process unit has been withdrawn to a position at which the nearest toner cartridge can be removed.