PURPOSE: To prevent the occurrence of cavitation, by a method wherein oil chambers across a cylinder to support a load are communicated to oil chambers across a compensation cylinder to which a means to exert a spring force in any displacement of a piston is mounted, and a ratio between the areas ot corresponding oil chambers of the one of the two cylinders is coincided with that of the other.

CONSTITUTION: Oil chambers A and B across a tilt cylinder 1 are communicated to an accumulator 2 through check valves 13 and 14. With the check valves 13 and 14 forcibly opened by means of a handle 15, the piston ot the cylinder 1 is displaced to an arbitrary position, and is fixed in a closed state. A compensate cylinder 3 is partitioned into oil chambers C and D on both sides by means of a piston 21, a spring force is exerted in any displacement of the piston 21 through the forces of springs 29 and 30, and the oil chamber C is communicated to the A and the oil chamber D to the B. An effective sectional area is set so that A/C is equal to B/C, and thus, during the overload action of the cylinder 1, cavitation due to deficiency in suction is prevented from occurring to the oil chamber on the expansion side of the cylinder 1.