An image forming device comprising: a frame comprising a pair of metal frames providing lateral side panels, each metal frame having an end face functioning as a first positioning part; an image forming unit detachably supported to the frame and having a support shaft engageable with the end face and functioning as a second positioning part, the second positioning part abuttable on the first positioning part for positioning the image forming unit relative to the frame; a door movable between an open position and a closed position relative to the frame, the image forming unit being detachable from or attachable to the frame at the open position; and a lock unit lockingly engagable with the door at the closed position of the door for maintaining the closed position, the lock unit being supported to the metal frame; and a pressure portion provided at the lock unit and configured to urge the image forming unit so as to press the second positioning part onto the first positioning part at the closed position of the door, the pressure portion urging the support shaft toward the end face; characterized in that the lock unit is movable between an upper position and a lower position at which the door is locked, and the image forming device comprises a cam member having a cam surface with which the lock unit is in contact, the cam surface being configured to move the lock unit toward the upper position in response to the movement of the door to the open position and to move the lock unit toward the lower position in response to the movement of the door to the closed position, the pressure portion being positioned above the end face and being movable toward the end face as a result of the movement of the lock unit toward the lower position, whereby the support shaft is pressed between the pressure portion and the end face in response to the movement of the door to the closed position. a frame comprising a pair of metal frames providing lateral side panels, each metal frame having an end face functioning as a first positioning part; an image forming unit detachably supported to the frame and having a support shaft engageable with the end face and functioning as a second positioning part, the second positioning part abuttable on the first positioning part for positioning the image forming unit relative to the frame; a door movable between an open position and a closed position relative to the frame, the image forming unit being detachable from or attachable to the frame at the open position; and a lock unit lockingly engagable with the door at the closed position of the door for maintaining the closed position, the lock unit being supported to the metal frame; and a pressure portion provided at the lock unit and configured to urge the image forming unit so as to press the second positioning part onto the first positioning part at the closed position of the door, the pressure portion urging the support shaft toward the end face; characterized in that the lock unit is movable between an upper position and a lower position at which the door is locked, and the image forming device comprises a cam member having a cam surface with which the lock unit is in contact, the cam surface being configured to move the lock unit toward the upper position in response to the movement of the door to the open position and to move the lock unit toward the lower position in response to the movement of the door to the closed position, the pressure portion being positioned above the end face and being movable toward the end face as a result of the movement of the lock unit toward the lower position, whereby the support shaft is pressed between the pressure portion and the end face in response to the movement of the door to the closed position.