To expedite the opening of a relief valve 11 in a case where the relief valve 11 and a relief hole 12 are disposed in a central portion of an elastic membrane 10.

A relief valve 11 is disposed at the center of an elastic membrane 10, and a relief hole 12 is disposed in a central portion thereof. The relief valve 11 is thinner than a body portion 14 on an outer circumferential side, and suppression projections 13 project at appropriate intervals in a circumferential direction on an upper surface of the relief valve 11. The elastic membrane 10 is integrated across an upper holder 20 and a lower holder 30 from above and below, and, at this time, the suppression projections 13 are in contact with projection support portions 23, and a central hole 22 of a central portion 21 is placed on the relief hole 12. A lower surface of the relief valve 11 is in close contact with a seal portion 31 of the lower holder 30, and the relief hole 12 is closed by the seal portion 31. When a negative pressure side is excessively deformed, lead deformation is performed so that a working fluid in an auxiliary liquid chamber 6 turns over an inner circumferential portion of the relief valve 11, the relief valve 11 is released from the seal portion 31 in a swift manner, and a swift leak is formed from the central hole 22 to a main liquid chamber 5.