PURPOSE: To apply engine brake so as to secure safety by forming the operating area of the piston of one hydraulic clutch larger than that of the other and providing an elastic body at the small area side piston.

CONSTITUTION: The first hydraulic clutch 7 for running and the secondary hydraulic clutch 9 are parallely provided in an axial direction in a common prime moving disk 5. Pistons 11, 12 are operated in a reverse direction mutually to be connected in an axial direction together. In both pistons 11, 12, one of the operating hydraulic area is formed large and the other is formed small, and an elastic body is provided at a small area side piston 12. Thus, engine brake is operated so as to obtain safety and its operation can be achieved smoothly and securely.