PURPOSE: To keep off any possible vibratory transmission in an extensive frequency range so effectively in a simple structure.

CONSTITUTION: This vibration isolator is provided with a main liquid chamber A, using a rubber elastic body 8, supporting a vibrator, as a chamber wall, and a sub-liquid chamber B being partitioned by the former and a partition plate 82 and using a rubber film 9 as the chamber wall, and these liquid chambers A, B are interconnected to each other by a passage 3, while a valve gear 4, altering the passage sectional area S, is installed in this passage 3. A control circuit 7 integrates each output signal of vibro-sensors 5, 6 installed in an upper plate 1 and a bottom plate 2 each, obtaining each of displacement velocities Xv, Xov of the vibrator 1 and the base body 2, and opening of the valve gear 4 is controlled to change the passage sectional area S in relation to an equation S=K1.Xv/(Xv-Xov) in setting K1 down to a proportional constant from these displacement velocities Xv, Xov. Thus, vibro-transmissibility is gradually lowered to the high frequency side irrespective of magnitude of a system damping coefficient, without entailing a peak of the vibro- transmissibility at a low frequency range.