PURPOSE: To give a driver no physical disorder by providing a plurality of viscous couplings between input and output shafts, and also providing a means which changes transmission characteristic of torque with equal percentage and selectively connects such couplings.

CONSTITUTION: When a selector knob 55b is moved, selective connection of a first or a second viscous coupling (hereinafter referred to as VC1, VC2) in a torque varying means 50. In the case that the selector knob 55b is on a position of P, connection is carried out at portions designated by mark. An output member O1 is connected to an input member I2, while VC1 and VC2 are in serial viscous arrangement. As transmission coefficients of VC1 and VC2 are the same, a torque Tr is half as in the case that only VC1 is used. In the case that the knob 55b is at a position P3, both of the input members I1 and I2 are connected to each other, and the torque Tr is the same as the torque of VC1. In the case that the knob 55b is on the position of P4, the members I1 and I2, O1 and O2 are respectively connected to each other and in a parallel viscous arrangement. The torque Tr in this case is twice as that of VC1. Torque transmission characteristic is thus changed with equal percentage, thereby an operator feels no physical disorder.