PURPOSE: To prevent the tight corner braking, by making a one end wall of a casing be axially movable by external operation, and thereby changing the torque transmitting characteristics.

CONSTITUTION: In the case of selecting a hump mode from a viscous mode, a fork 16 is moved in the direction of arrow a to thereby outwardly raise a moving piece 9 along a guide groove 10 by means of a link 17 and axially inwardly urge a piston 8 through an inclined surface 11. An amount of movement of the piston 8 is set to be within a compressible area of compressive fluid stored in a viscous coupling A. Accordingly, the movement of the piston 8 causes an increase in inner pressure to change a viscous fluid density. As a result, a transmitting torque is increased to change the characteristics of the viscous mode. Therefore, it is possible to prevent tight corner braking under a normal condition.