An image forming device has: a developing cartridge for developing an electrostatic latent image on an image carrying member and a toner cartridge. The developing cartridge contains a developing roller and a single auger. The toner cartridge is detachably attached to the developing cartridge and containing toner. The toner cartridge has an agitation mechanism that agitates the toner, a conveyance mechanism that conveys the toner to the agitation mechanism, and two ports, the two ports being aligned in the axial direction and communicated with the developing cartridge, respectively. The toner cartridge and the developing cartridge are arrayed side by side in a horizontal direction. The toner cartridge has a shape having a vertical cross section in which a horizontal length is greater than a vertical length.