An image forming device includes: a main casing; a retaining member; a plurality of developing units; a plurality of toner cartridges; and an operation unit. The retaining member is configured to be movable in a first direction between an accommodated position and a pulled-out position. The retaining member includes a first side plate and a second side plate extending in the first direction and disposed in confrontation with and spaced apart from each other in a second direction orthogonal to the first direction. The developing units are retained in the retaining member. The toner cartridges have one-on-one correspondence to the developing units and are configured to be mountable in or detachable from the retaining member when the retaining member is located at the pulled-out position. Each toner cartridge includes a casing formed with a cartridge opening, and a cartridge shutter configured to be movable between an open position for opening the cartridge opening and a closed position for closing the cartridge opening. The operation unit is provided in the retaining member and configured to be pivotally movable in an imaginary plane parallel to the first direction and the second direction. The operation unit is interlocked with the cartridge shutter while the toner cartridge is mounted in the retaining member to move the cartridge shutter between the open position and the closed position in accordance with the pivotal movement of the operation unit.