In the breakdown preventive mechanism of a rotary damper, when the upper rotor is rotated in one direction, a communication path is formed between the valve body and the lower rotor, and the pressure in the pressure chamber is formed between the lower rotor and the lower housing is lowered, and rotation is easily performed. When the upper rotor is rotated in another direction, the viscous fluid does not flow between the valve body and the lower rotor, and the pressure in the pressure chamber formed between the lower rotor and the lower housing is raised. As a result, the viscous fluid pushes down the relief valve and a cage type relief valve by resisting the elastic element, and thereby prevents breakdown of the rotary damper.