The present invention is developing device itself is to be directly mounted to the printer body while using the top cover substitutes and, by a simple locking device when removably hanging, the removal of the developing cartridge is very simple hameuroseo to be unlocked and that the toner is the convenience to the user in hand at the same time relates to a developing cartridge detachable in a laser beam printer to assist in cost reduction devices, at the top of the printer body 1 to the lower portion by inserting the developing cartridge 2, the upper surface is one in correspondence with the upper surface of the Freeter body 1 the handle is behind the form to issue lift to the upper jaw (3) and a developing device which is incorporated in the lower portion of the developing cartridge (2) is provided so as to rotate by frictional force between the transfer roller 4 of the printer body (1) so as to supply the print drum a locking button to 5 and, with the locking projection 6 which projects laterally on one side of the developing cartridge (2), is provided to be exposed upward in the printer main body 1 up, down operation ( 7) and a locking button (7) the hinge pin (and 9), the center is operated seesaw the locking lever (8) for hanging, unlock the locking protrusion 6 of the developing device (2), the locking button (7) by and a first elastic member 10 which is provided to give to maintain the horizontal balance resiliently to the locking lever (8), was given to incorporated in the printer main body 1 beneath resiliently the bottom surface when mounted in the developing device 2, the developing cartridge drum ( 5) constitute a developing cartridge detachable device of a laser beam printer as a second resilient member (11) for so adjusting the pressure between the optical transfer roller (4).