PURPOSE: To make it possible to travel at a constant low vibration level, to prevent an occurence of car shake and to prevent a muffled noise by rendering the spring constant of a resilient body to be smaller when the vehicle speed exceeds a predetermined value.

CONSTITUTION: A first rubber insulator 25 is interposed between a vehicle body side member 19 and an upper plate 21. A second rubber insulator 29 is interposed between the upper plate 21 and a lower plate. A third rubber insulator 33 is interposed between the lower plate 26 and a disc 31. A pair of cams 37 is fixed to a hinge pin 38 which is rotatably supported to a frame side member 11. When the vehicle speed exceeds a predetermined value, a cam actuating device 40 rotates the hinge pin 38 in response to a signal from a control means 44 to tightly contact the lower plate 26 on a lower wall 13, and at the same time, a projection 27 is engaged with a depression 17 to make the spring constant of a resilient body 35 to be small.