To generate a high braking torque in time of braking as well as to secure a stable braking characteristic in time of non-braking by using an electroviscous fluid where one side of the shaft members is inserted through a fluid storage tank, and the power is transmitted to the other side one, and installing a stirring means in space between two transmitting plates.

This power transmission is equipped with a fluid storage tank 11 filled up with an electroviscous fluid whose shearing stress (viscosity) varies according to voltage to be impressed from the outside, each of members 13 and 14 inserted with one side of a shaft into a tank wall 11a, each of transmitting plates 16 and 17 installed tight in each shaft end of these shaft members 13 and 14 so as to stand opposite to each other holding the electroviscous fluid between, two electrode parts 18 and 19 installed on these transmittal plates 16 and 17 so as to stand opposite to each other holding this electroviscous fluid between, a voltage generator 21 controlling any voltage to be impressed on an interval between both these electrode parts 18 and 19, and a stirring means 23 forming a flow of the electroviscous fluid, respectively. With this constitution, in time of braking operation to impress voltage on these electrode parts 18 and 19, high braking torque is generated, and in time of non-braking to impress no voltage, an excellent braking characteristic, where excessive load is not imposed on, is thus stably securable.