A plate cylinder drive unit in a printer which rotatably drives a plate cylinder by transmitting rotation of a drive helical gear to a plate cylinder comprising: a plate cylinder drive shaft rotatably supported on a frame of the printer and connected to the plate cylinder to rotate integrally with the plate cylinder; a driven gear member having a driven helical gear that meshes with the drive helical gear and attached around the plate cylinder drive shaft in such a manner that the driven gear member is rotatable and axially movable with respect to the plate cylinder drive shaft; register means for axially moving the driven gear member with respect to the plate cylinder drive shaft and fixing the driven gear member at a predetermined position; a whirl-stop member attached to the plate cylinder drive shaft in such a manner that the whirl-stop member is nonrotatable but axially movable with respect to the plate cylinder drive shaft; whirl-stop-member fixation means for removably fixing the whirl-stop member to the driven gear member; characterized in that the plate cylinder drive unit further comprises: a backlash eliminating helical gear that meshes with the drive helical gear and is disposed in contact with the driven helical gear on one side of the driven helical gear in such a manner that the backlash eliminating helical gear is rotatable and axially movable within a predetermined range with respect to the driven helical gear; a first elastic member biasing the backlash eliminating helical gear toward the driven helical gear to bring the backlash eliminating helical gear in pressure contact with the one side of the driven helical gear; and a second elastic member biasing the backlash eliminating helical gear in such a manner that the backlash eliminating helical gear rotates in a predetermined direction with respect to the driven helical gear. a plate cylinder drive shaft rotatably supported on a frame of the printer and connected to the plate cylinder to rotate integrally with the plate cylinder; a driven gear member having a driven helical gear that meshes with the drive helical gear and attached around the plate cylinder drive shaft in such a manner that the driven gear member is rotatable and axially movable with respect to the plate cylinder drive shaft; register means for axially moving the driven gear member with respect to the plate cylinder drive shaft and fixing the driven gear member at a predetermined position; a whirl-stop member attached to the plate cylinder drive shaft in such a manner that the whirl-stop member is nonrotatable but axially movable with respect to the plate cylinder drive shaft; whirl-stop-member fixation means for removably fixing the whirl-stop member to the driven gear member; characterized in that the plate cylinder drive unit further comprises: a backlash eliminating helical gear that meshes with the drive helical gear and is disposed in contact with the driven helical gear on one side of the driven helical gear in such a manner that the backlash eliminating helical gear is rotatable and axially movable within a predetermined range with respect to the driven helical gear; a first elastic member biasing the backlash eliminating helical gear toward the driven helical gear to bring the backlash eliminating helical gear in pressure contact with the one side of the driven helical gear; and a second elastic member biasing the backlash eliminating helical gear in such a manner that the backlash eliminating helical gear rotates in a predetermined direction with respect to the driven helical gear. a backlash eliminating helical gear that meshes with the drive helical gear and is disposed in contact with the driven helical gear on one side of the driven helical gear in such a manner that the backlash eliminating helical gear is rotatable and axially movable within a predetermined range with respect to the driven helical gear; a first elastic member biasing the backlash eliminating helical gear toward the driven helical gear to bring the backlash eliminating helical gear in pressure contact with the one side of the driven helical gear; and a second elastic member biasing the backlash eliminating helical gear in such a manner that the backlash eliminating helical gear rotates in a predetermined direction with respect to the driven helical gear.