PURPOSE: To moderate vibration and shock by disposing a slip clutch at a lock-up clutch in series and providing a cam mechanism,for generating tension corresponding to load torque, at the output side power transmission path of a fluid coupling.

CONSTITUTION: A slip clutch 5 is series-disposed at a lock-up clutch 3 for connecting the output side 21 of a fluid coupling 2 directly to the input side 22, so that large load torque is absorbed by the slip of the clutch 5. A cam mechanism 6 for generating tension corresponding to the load torque is disposed at a power transmission path in the output side 21. When the clutch 5 slips, the fluid coupling 2 is relatively rotated on the input-output sides to perform transmission through a turbine runner 21a, the cam mechanism 6 and a hub part 21b. In this case, tension corresponding to the load torque is generated at the cam mechanism by the relative rotation, and the slip clutch 5 enlarges torque capacity correspondingly, thus improving drivability.