A first connecting member 1 secured to an engine, a second connecting member connected to an automobile vehicle and a rubber block 3 of a cylindrical shape for connecting the first and second connecting members are provided. The rubber block 3 is divided by a partition wall 6 into a first chamber 8 and a second chamber 9 and an orifice passage 34 is disposed to communicate with both chambers. The orifice passage 34 and the first chamber 8 are adapted to communicate with each other by an intermediate opening portion 37 which is opened and closed by a passage length adjustable valve 38 and passage end opening portions which are always open. The rubber block 3 has a cylindrical portion 5, the wall of which serves at a part thereof as a movable wall portion 12. When a control chamber 26 provided on the periphery is kept under negative pressure, the internal pressure of the first chamber 8 is lowered and the passage length adjustable valve 38 integrally formed with the movable wall portion 12 starts its displacement to open the intermediate opening portion 37, wherein the orifice length is shortened. Thus, it is possible to set a dynamic spring minimum value in an idling vibration range and remarkably reduce the dynamic spring rate.