PURPOSE: To enable to facilitate displacement control by a method wherein the relative displacement between the power unit and a car body is precisely detected in order to supply and discharge fluid to and from bag-shape members provided between the power unit and the car body for displacement control.

CONSTITUTION: The fluid-filled elastic bag-shape members 7 and 9 are mounted between the power unit and the car body and the fluid is supplied to and discharged from the bag-shape members 7 and 9 by means of a fluid control means so as to detect the relative displacement and consequently to keep the displacement within a certain fixed range. Due to the structure that one pair of the opposing parts of power unit side and of car body side approach each other, while the other pair part from each other by means of the relative displacement in the direction intersecting the direction for detecting the displacement between the power unit and the car body, the respective relative displacements in the direction for detecting the displacement between the power unit and the car body are detected and a summed displacement signal added up the detected displacement signals is outputted.