To obtain a unidirectional rotary speed-reducing device which spares space and reduces the number of parts.

This device is provided with a housing 22 in which a viscous fluid is contained and hermetically sealed, a rotor 28 rotatably arranged against the housing 22, and vanes 30 fixed to the nearby position of the edge of the rotary body 28a of the rotor 28, at the one side thereof. The vanes 30 are turned to the right angled position by one of either concentrical clockwise or anticlockwise stream concentrical to the rotor shaft 28b, of the viscous fluid generated by the relative rotation between the rotor 28 and the housing 22, and the fluid resistance of the viscous fluid is increased to increase the damping capacity. And the vanes 30 are turned to the nearly horizontal position by the other of either clockwise or anticlockwise stream concentrical to the rotor shaft 28b, of the viscous fluid generated by the reverse relative rotation between the rotor 28 and the housing 22 and the fluid resistance of the viscous fluid is decreased to reduce the damping capacity.