An image forming apparatus comprising: a main casing; a photosensitive drum rotatable about a drum axis extending in a first direction; a developing cartridge comprising a toner casing that can accommodate therein toner, the developing cartridge supplying the toner onto the photosensitive drum, the developing cartridge being detachably attachable to the main casing; a transfer unit for transferring the toner that has been supplied onto the photosensitive drum to a sheet; a solvent cartridge comprising a solvent casing that can accommodate therein solvent for dissolving the toner, the solvent cartridge being detachably attachable to the main casing independently of the developing cartridge; an applicator applying the solvent to the toner that has been transferred to the sheet; and a heater heating the sheet on which the solvent has been applied to the toner by the applicator, wherein the main casing has one end portion (2E1) and another end portion (2E2) in a second direction, the second direction crossing the first direction, the another end portion (2E2) being spaced apart from the one end portion (2E1) in the second direction, wherein the applicator is positioned at the one end portion (2E1) of the main casing, wherein the main casing has an opening through which the developing cartridge and the solvent cartridge pass when the developing cartridge and the solvent cartridge are attached to the main casing, the opening being positioned at the another end portion (2E2), the image forming apparatus further comprising a cover positioned at the another end portion (2E2), the cover being movable between: a closed position where the cover closes the opening; and an open position where the opening is opened, wherein the developing cartridge and the solvent cartridge are positioned between the applicator and the cover in the second direction, and the solvent cartridge is positioned closer to the applicator than the developing cartridge is to the applicator in a state where the developing cartridge and the solvent cartridge are attached to the main casing and the cover is at the closed position. a main casing; a photosensitive drum rotatable about a drum axis extending in a first direction; a developing cartridge comprising a toner casing that can accommodate therein toner, the developing cartridge supplying the toner onto the photosensitive drum, the developing cartridge being detachably attachable to the main casing; a transfer unit for transferring the toner that has been supplied onto the photosensitive drum to a sheet; a solvent cartridge comprising a solvent casing that can accommodate therein solvent for dissolving the toner, the solvent cartridge being detachably attachable to the main casing independently of the developing cartridge; an applicator applying the solvent to the toner that has been transferred to the sheet; and a heater heating the sheet on which the solvent has been applied to the toner by the applicator, wherein the main casing has one end portion (2E1) and another end portion (2E2) in a second direction, the second direction crossing the first direction, the another end portion (2E2) being spaced apart from the one end portion (2E1) in the second direction, wherein the applicator is positioned at the one end portion (2E1) of the main casing, wherein the main casing has an opening through which the developing cartridge and the solvent cartridge pass when the developing cartridge and the solvent cartridge are attached to the main casing, the opening being positioned at the another end portion (2E2), the image forming apparatus further comprising a cover positioned at the another end portion (2E2), the cover being movable between: a closed position where the cover closes the opening; and an open position where the opening is opened, a closed position where the cover closes the opening; and an open position where the opening is opened, wherein the developing cartridge and the solvent cartridge are positioned between the applicator and the cover in the second direction, and the solvent cartridge is positioned closer to the applicator than the developing cartridge is to the applicator in a state where the developing cartridge and the solvent cartridge are attached to the main casing and the cover is at the closed position.