PURPOSE: To facilitate maintenance by arranging a steering clutch in a transmission case, a steering brake, and a spin turn clutch to the outside in an axial direction of the steering clutch shaft and a spin clutch shaft.

CONSTITUTION: A gear mechanism 5 is arranged to a central part in an axial direction between a steering clutch shaft 3 and a spin turn clutch shaft 4 so as to effect gear transmission from a gear shifting device 1 to a pair of ring t and left running drive shafts 2. On both sides of a gear mechanism 5, a steering clutch 8 comprising clutch packs 6 and 7 in a fictional mode through hydraulic operation, a steering brake 9, and a spin turn clutch 10 the steering side of which is reversely transmitted are arranged on the steering clutch shaft 3 and the spin turn clutch shaft 4. By effecting covering and uncovering by means of covers 12 and 13 of a transmission case 11 at the end parts of the shafts 3 and 4, so that opening and closing may take place.