In an elastic partition member, a movable diaphragm outer circumferential section for supporting an outer circumference of an elastic movable diaphragm is formed integral with a relief valve. The elastic movable diaphragm is integrally formed with the elastic movable diaphragm. An upward circular arc projection that extends upwardly is provided on an upper part of the movable diaphragm outer circumferential section, and is fixed by a fixing portion provided in an upper member of a partition. A portion in the vicinity of a base portion of the relief valve is formed as a rectilinear unsupported portion that is flush with the elastic movable diaphragm. The base portion extends rectilinearly along the unsupported portion. When being opened, the relief valve is easily bent by the rectilinear base portion and is elastically deformable large enough to extend above the unsupported portion.