An image forming apparatus, including: a housing; a photoconductor; a developing unit configured to form a developer image on the photoconductor; a developer cartridge storing developer, the cartridge being removably mountable on the housing, a supplier configured to supply the developer from the developer cartridge to the developing unit; and a controller configured to execute a first printing process of forming a developer image by use of the developer supplied from the developer cartridge, when the developer cartridge is being mounted on the housing, and a second printing process of forming a developer image by use of the developer remaining in the developing unit, when a usage amount of the developing unit is not less than a predetermined value and the developer cartridge is not being mounted on the housing, wherein the controller does not execute the second printing process when the usage amount is less than the predetermined value.