To prevent a piston from being moved by external force caused by vibration and to prevent the operation mistake of a liftable car deck by combining a release stop metal at the tip of a support pin and a spring cylinder.

In the holding state, a selector valve 25 is pushed by the spring force of a spring cylinder 2 in a return port, a release stop metal 3 at the tip of a support pin 9 is locked to the receiving metal 6 of a bracket 7 with the support pin 9 kept extruded, and the support pin 9 is mounted on the bracket 7. Even if the selector valve 25 is erroneously operated to a forward port in this state, the release stop metal 3 at the tip of the support pin 9 is locked to the receiving metal 6 stationarily, and a liftable car deck 33 is never dropped. In the lifting/lowering state, the liftable car deck 33 is temporarily hoisted by a hoisting device 4 to release the release stop metal 3 from the receiving metal 6, the selector valve 25 is set to the forward port, application force is applied to the spring cylinder 2 to release the support pin 9 from the bracket 7, and the liftable car deck 33 is moved to the prescribed position by the hoisting device 4.