To support driving force of an engine by recovering energy of exhaust gas, without imparting a large shock to a shift transmission device, by securing a relatively small space, by adding a relatively little part.

A power turbine 17 is arranged in an exhaust passage 14 on the exhaust gas downstream side of a main turbine 15b of a turbocharger 15, and a power shaft 17a of the power turbine and a crankshaft 10a are connected via a viscous coupling 21. A hub for a coupling is connected to the power shaft, and a case for a coupling is connected to the crankshaft, and a sealed space is formed by relatively rotatably inserting the hub for a coupling into the case for a coupling. An inner plate is relatively nonrotatably stored in the sealed space to the hub for a coupling, and an outer plate is stored in the sealed space so as to be relatively nonrotatable to the hub for a coupling and to be alternately arranged with the inner plate, and a hydraulic fluid is also sealed in the sealed space.