An image forming device having a frame, a door, and a movable image forming unit to be assembled in the frame when the door is open. The image forming unit has a support shaft, and frame has a hole on which the support shaft abuts for positioning of the image forming unit relative to the frame when the door is closed. A lock unit formed with a recessed portion is movable in association with the movement of the door. When the door is open, the lock unit is at its abutment position so that the support shaft abuts the recessed portion. When the door is closed the lock unit is moved so that the support shaft is brought into abutment with the hole.