It is an object of the present invention to provide a rotary damper capable of automatically adjusting an exhibited braking force in correspondence with variation in load.A fluid chamber 2 into which viscous fluid is charged is formed in a casing 1. A vane 3 is disposed in the fluid chamber 2. The vane 3 is formed with a fluid passage 5, and is provided with a valve 6. The valve 6 automatically varies a flow rate of the viscous fluid passing through the fluid passage 5 in correspondence with variation in load. With this structure, it is possible to automatically adjust the exhibited braking force in correspondence with variation in load caused by variation in rotational motion of a subject to be controlled, and to reduce variation in rotation speed of the subject to be controlled to an extremely small value.