A liquid-sealed antivibration device comprising: a first mounting member to be mounted on one of a vibration source side and a support side; a second mounting member to be mounted on the other of the vibration source side and the support side; a vibration-isolating base formed of a rubber-like elastic body interposed between the first mounting member and the second mounting member; a main liquid chamber where the vibration-isolating base forms a part of a chamber wall of the main liquid chamber and which is filled with a liquid; at least one subsidiary liquid chamber (34B, 34C) where a diaphragm formed of a rubber-like elastic body forms a part of a chamber wall of the subsidiary liquid chamber (34B, 34C) and which is filled with a liquid; a partition element by which the main liquid chamber and any one of the subsidiary liquid chambers (34B, 34C) are, partitioned from each other; and a first orifice flow passage which allows the main liquid chamber to be communicated with any one of the subsidiary liquid chambers (34B, 34C), wherein the partition element includes an elastic membrane, and the elastic membrane is made of rubber composition containing butyl rubber or halogenated butyl rubber, the liquid-sealed antivibration device includes a second orifice flow passage which is tuned to a frequency band higher than a frequency band of the first orifice flow passage and allows two liquid chambers out of the main liquid chamber and subsidiary liquid chambers (34B, 34C) to be communicated with each other, and the second orifice flow passage is formed in the partition element, characterised in that the elastic membrane is formed as a valve element for opening and closing the second orifice flow passage, a valve accommodating chamber is formed in the partition element in such a manner that the valve accommodating chamber accommodates and holds the elastic membrane in a portion of the second orifice flow passage in a direction orthogonal to a flow direction of the flow passage, the elastic membrane is held at an outer peripheral portion thereof between wall surfaces of the valve accommodating chamber, and includes inside the outer peripheral portion thereof a flexible membrane portion configured to be subjected to flexural deformation by liquid flow in the second orifice flow passage and thereby close an opening (160C, 160D) of the second orifice flow passage to the valve accommodating chamber provided in the partition element, and the membrane portion has a communication hole through which the second orifice flow passage is communicated at a position where the membrane portion does not overlap with the opening formed in the partition element, and the second orifice flow passage is released in a state where the membrane portion is spaced apart from the opening (160C, 160D). a first mounting member to be mounted on one of a vibration source side and a support side; a second mounting member to be mounted on the other of the vibration source side and the support side; a vibration-isolating base formed of a rubber-like elastic body interposed between the first mounting member and the second mounting member; a main liquid chamber where the vibration-isolating base forms a part of a chamber wall of the main liquid chamber and which is filled with a liquid; at least one subsidiary liquid chamber (34B, 34C) where a diaphragm formed of a rubber-like elastic body forms a part of a chamber wall of the subsidiary liquid chamber (34B, 34C) and which is filled with a liquid; a partition element by which the main liquid chamber and any one of the subsidiary liquid chambers (34B, 34C) are, partitioned from each other; and a first orifice flow passage which allows the main liquid chamber to be communicated with any one of the subsidiary liquid chambers (34B, 34C), wherein the partition element includes an elastic membrane, and the elastic membrane is made of rubber composition containing butyl rubber or halogenated butyl rubber, the liquid-sealed antivibration device includes a second orifice flow passage which is tuned to a frequency band higher than a frequency band of the first orifice flow passage and allows two liquid chambers out of the main liquid chamber and subsidiary liquid chambers (34B, 34C) to be communicated with each other, and the second orifice flow passage is formed in the partition element, characterised in that the elastic membrane is formed as a valve element for opening and closing the second orifice flow passage, a valve accommodating chamber is formed in the partition element in such a manner that the valve accommodating chamber accommodates and holds the elastic membrane in a portion of the second orifice flow passage in a direction orthogonal to a flow direction of the flow passage, the elastic membrane is held at an outer peripheral portion thereof between wall surfaces of the valve accommodating chamber, and includes inside the outer peripheral portion thereof a flexible membrane portion configured to be subjected to flexural deformation by liquid flow in the second orifice flow passage and thereby close an opening (160C, 160D) of the second orifice flow passage to the valve accommodating chamber provided in the partition element, and the membrane portion has a communication hole through which the second orifice flow passage is communicated at a position where the membrane portion does not overlap with the opening formed in the partition element, and the second orifice flow passage is released in a state where the membrane portion is spaced apart from the opening (160C, 160D).