PURPOSE: To prevent permeation of gear oil to gas and a disk and carry out normal brake operation at all times.

CONSTITUTION: A circular plate 20 is inserted into a stepped part provided on an input shaft 7 and fixed thereto by a boss which is shrinkage fitted therein. A recessed part 22 whose bottom surface 2a is inclined so as to descend toward a gear case 11 side on the bottom part of an inner circumferential surface, and a vertical shaped groove 23 whose top part is communicated with a recessed part 22 on the side surface of the gear case 11 side and whose bottom part is opened to the outer circumferential surface of a frame 21, are provided in a frame 21. An oil receiving box 24 provided with an opening part communicated with the bottom part of the groove 23 is provided on the bottom part of the outer circumferential surface of the frame 21.