The amounts of displacement of trunnions installed in first and second cavities are made equal to each other irrespective of the amount of resilient deformation. Also, if the worst should happen, the stability of the running of an automobile is secured. The supply and discharge of pressure oil to first and second actuators for displacing the trunnions installed in the first and second cavities are effected by first and second control valves independent of each other. A value is included for controlling so that the difference between oil pressure fed into the first actuator and oil pressure fed into the second actuator may not become great.