To enable a lock cylinder separated from an extensible element constituting a vibration damper to select the permission of the extension of the extensible element, and to assure the operation of an on-off valve.

A fluid pressure circuit includes the lock cylinder 1 which defines a piston-side chamber R2 which is set in a uniflow type, and filled with an operating fluid by a piston body 13 slidably accommodated in a cylinder body 11, and a rod-side chamber R1, connected to a fixed side at one end side, and connected to a movable side at the other end side. The piston-side chamber R2 is made to communicate with the rod-side chamber R1 under the arrangement of a check valve 2, a reservoir tank T is made to communicate with the piston-side chamber R2 under the arrangement of a check valve 3, and the rod-side chamber R1 is made to communicate with the reservoir tank T under the arrangement of a logic valve 4. The fluid pressure circuit includes an always-closed on-off valve 5 which switches a position to a communication position 5b by receiving the supply of pilot pressure from the logic valve 4, and switches the position to a shut-off position 5a by manual operation. The switch valve 5 switches the position to the communication position 5b, and thereby the logic valve 4 is opened and operated.