PURPOSE: To secure a hydrodynamic retarder unit whose durability is well improved since its operation is merely stopped as the clutch unit is cut off even in the case where a second bearing seizes tight and a movable member drags together with a turning shaft.

CONSTITUTION: In this retarder unit, a cam member 121 is rotated by a driving device, and a moved member 120 is also rotated by a cam part 121b of this cam member 121 engaging with an engaging part 120b, whereby each ball is relatively shifted in a direction where the cross section of each wedgelike space 128 becomes lessened, and when the movable member 120 is pushed out in the central axial direction and connected to a clutch unit, a turning direction of this movable member 120 is set to the opposite direction to a rotational direction B at a normal condition of a turning shaft 10.