A developing device includes a developing agent carrying body that carries toner, a toner supply member that supplies toner to the developing agent carrying body, a regulating member that regulates an amount of toner present on the developing agent carrying body, a developing chamber having the developing agent carrying body, the toner supply member and the regulating member, and a replaceable toner cartridge that supplies toner to the developing chamber and collects the toner from the developing chamber. When the toner cartridge is replaced with a toner cartridge storing new toner, the new toner is supplied to the developing chamber from the toner cartridge. The new toner and the toner remaining in the developing chamber are agitated to form agitated toner, and the agitated toner is collected in the toner cartridge from the developing chamber.