Provided is an image forming apparatus. The image forming apparatus includes a print unit printing having a plurality of developer cartridges, each of which has a photoreceptor drum disposed therein. A plurality of fixing members are movable between a first position where the developer cartridges are installed and removed and a second position where the developer cartridges are fixed to the print unit. A pinion includes a female screw portion and a first sliding member includes a male screw portion that is engaged with the female screw portion, sliding backwards and forwards as the pinion rotates, and moving the fixing members to the first position or the second position.