In a new printer, a toner cartridge is mounted in the state where there is no toner in a reserve tank, and toner of the amount of filling toner D therein is supplied with the amount of full accommodation toner E for filling up the reserve tank by a toner supply mechanism. The toner of the amount of remaining surplus toner F remains in the toner cartridge. The amount of toner consumption G is measured by a dot counter during a printing execution. In the case where a toner density sensor detects a toner decreasing, the toner is supplied to a developing unit from the reserve tank. When it becomes G>F, it display-informs a user of “toner cartridge exchange is allowed”. Hereby, it enables to exchange the toner cartridge without stopping operation of the main body of the image forming apparatus using a two-component developer.