An image forming device includes a main casing, a support member, a plurality of image bearing members supported on the support member, a plurality of developing units supported on the support member, a plurality of toner cartridges detachably mounted on the support member, and a shutter closing mechanism. Each toner cartridge includes a casing formed with an opening through which toner accommodated in the casing is supplied to the developing unit and a shutter movable between an opening position at which the shutter opens the opening and a closing position at which the shutter closes the opening. The shutter closing mechanism moves the shutter of one of the toner cartridges to be replaced from the opening position to the closing position while maintaining the shutters of the remaining toner cartridges at the respective opening positions.