The front sides of pinions are rotatably supported by a first case via first taper roller bearings, and the back sides of the pinions are rotatably supported by a second case via second taper roller bearings, and the back sides of input shafts are rotatably supported by a case cover via third taper roller bearings, and clutches and input gears are stored in the case cover, and sleeves are interposed between the first taper roller bearings and the front side of the pinions, and the pinions are configured to be inserted and extracted into/from the sleeves.