To prevent an oil sump from running out through a sump conduit pipe and to improve reliability by providing a means to provide balance of pressure of force working on a level of a working means sump (main chamber) at the time of braking driving.

A retarder 1 has a rotor and a stator and is furnished with a retarder housing 2 having a main chamber 2.1 and an auxiliary chamber 2.4 on which an oil sump is provided. A partition wall 2.2 to separate the main chamber 2.1 from a rising passage 2.3 is provided in the housing 2. A proportional valve 3 is provided as a control device, connected to the main chamber 2.1 by a conduit pipe 3.1 and is also connected to an oil auxiliary chamber 3.3 by a conduit pipe 3.2. Additionally, a heat exchanger 4 is connected to a working space of the retarder 1 through a conduit pipe 4.1, and throttle valves 1.2, 2/2 valve 1.3 are provided in a passage connected to a clearance between the rotor and the stator. The valve 1.3 is free to be connected to the main chamber 2.1 through an outlet 1.4 and also free to be connected to a 3/2 root valve through a conduit pipe 1.5.