PURPOSE: To quickly perform an operation without a circulating pump by disposing each of conduits of different diameters among entrance and exit of a retarder and a heat exchange, separating the conduits just after the exit of the retarder and joining the conduits just before the entrance and providing a check valve on the conduit on a thick side.
CONSTITUTION: An exit opening 6 of a retarder 1 having a rotary blade 2 and a fixed blade 3 is connected with cooling circulating conduits 10, 11 branching at a branching point 8 just after the retarder 1. The conduit 10 is 80% of cross section and the conduit 11 has 20% and lead to a heat exchanger 12. The conduit 10 leads from the heat exchange 12 to the retarder 1 and joins the conduit 11 at a joining point 9. The conduit 10 having larger cross section is provided with check valves 31, 32. Working fluid enters from a collecting tank 16 to the conduit 10 and the retarder 1 by compressed air from a conduit 19 and goes to the heat exchange 12. Because the check valves 31, 32 and a valve piston 23 of a control valve 22 of the conduit 11 are operated and a circulating path is filled with working fluid also after a braking process is terminated, braking force is quickly generated and a sure operation is performed without a circulating pump.