A piston rod is connected to a piston and extends through a seal in the end face of a cylinder. A locking tube which surrounds the cylinder when the piston rod is retracted has a first end section mounted to the free end of the piston rod so that the locking tube is pivotable between a release position, wherein the locking tube is substantially coaxial to the piston rod, and a locking position, wherein the locking tube is inclined toward the piston rod. An opposed second end section has a supporting stop which can be positioned axially against the free end of the cylinder when the piston rod is extended and the locking tube is in the locking position. An actuable retaining device retains the locking tube in the locking position only by manually pivoting the locking tube into the locking position when the piston rod is extended.