To form a relief valve for preventing the generation of the cavitation phenomenon into a simple and accurately operable structure, there are provided a central thin wall portion and a fixing portion in an elastic diaphragm provided in the partition member to have the fixing portion fixed in position. A relief valve is integrally formed with an outer peripheral portion of the fixing portion. The relief valve is provided with an inclined surface formed on the side of a secondary liquid chamber and a recessed portion opened on the side of a primary liquid chamber and has a difference in rigidity in the circumferential direction. When the primary liquid chamber turns negative pressure, a hydraulic liquid in the secondary liquid chamber opens the relief valve and leaks to the primary liquid chamber so as to prevent the generation of the cavitation phenomenon.