PURPOSE: To prevent excessive force from acting upon a casing and other components or joint parts due to the expansion of an operating fluid in association with temperature rise.

CONSTITUTION: An auxiliary fluid chamber 3 is formed at the partition part of a casing, and the chamber 3 is communicated with a chamber filled with an operating fluid by a groove 6 formed at a flow member 7. A piston 15 and a spring 16 are disposed in the auxiliary fluid chamber 3, and the piston 15 is energized onto the side of the groove 6 and a flow hole 14. When the temperature of a rotary damper rises to generate the cubical expansion of the operating fluid, this operating fluid flows into the auxiliary fluid chamber 3 from its chamber through the groove 6. Pressure rise caused by the expansion of the operating fluid is thereby prevented.