PURPOSE: To improve the traveling performance of a vehicle by constituting a viscous resistance force generating means from a viscous fluid chamber and the first and second fins and installing one cam member in slidable manners to the inside/outside of the fluid chamber and sealing the inside of the fluid chamber with each prescribed quantity of viscous fluid and gas.

CONSTITUTION: As for a power transmission device 10, one cam member 14 is gradually slided to a fluid chamber R side by the action of a roller 17 as a cam follower, according to the increase of the differential revolution speed between the both propeller shafts 25 and 26, and the superposition length of the fin parts in the both fins 14a and 15a is increased, and the charging rate of the viscous fluid in the fluid chamber R is increased. Therefore, the transmission torque is increased according to the increase of the both, and catches the reduction of the transmission torque caused by the reduction of viscosity of the viscous fluid, or overwhelms the reduction of the transmission torque. Therefore, in the power transmission device 10, the torque transmission characteristic is set close to the lower limit of the region for generating tight corner braking phenomenon, and the traveling performance of a vehicle can be improved.