PURPOSE: To simplify piping connection required for a below-mentioned mechanisms, by a method wherein, in a single acting cylinder which tilts a load box of a damp car, a by-pass mechanism, which functions when it stretches to a full length, and a throttle mechanism, which operates when it contracts to a minimum length, are monolithically built in a main body.

CONSTITUTION: Hydraulic oil, which is fed to a main body 2 from a high pressure path 9 in order to stretch a cylinder 1 from such a state as causes it to contract to its minimum length, flows to a connection path 21, and presses a throttle valve 33 against a cylinder (27) side against a spring 36. Thereby, the connection path 21 is released, the hydraulic oil flows into an inner cylinder 6, and a middle cylinder 5 and the inner cylinder 6 protrude against an outer cylinder 3. When the inner cylinder 6 stretches to approximately its full length, a support ring 30 makes contact with a cap 24, and a by-pass valve body 14 moves to the left against a spring 23, whereby a low-pressure path 10 is released and hydraulic oil is by-passed. Meanwhile, in the case of the cylinder 1 and when the cylinder 1 contracts to about its minimum length, a throttle valve body 21 is throttled, whereby it is reduced in a contraction rate.