PURPOSE: To increase the capacity of a retarder and improve durability, by providing an exhaust brake for a retarder brake circuit using an oil-cooled multi-plate type retarder wherein the exhaust brake works via a pressure switch and a solenoid valve.

CONSTITUTION: Air from a retarder air tank 9 is flowed from a pipe 11 via an auto reatarder valve 14 and a double check valve 13, and is streamed into relay valves 7, 7' via pipes 11a, 12a, 12, the double check valve 13, a pipe 16, and double check valves 15, 15' by the opening of a retarder valve 10. Then, the relay valves 7, 7' are opened, and rear brakes 1, 1' are operated as a retarder. In the above construction, the action of the retarder is detected by a pressure switch 17, so that an exhaust brake 21 is operated by a solenoid 19. Thus, the capacity of the retarder is increased, its efficiency is exhibited with a favorably responsible exhaust retarder on early stage.