A locking device is used for locking at least a piston associated with an operating element of a valve, a choke, a blowout preventer or some other unit used in the field of oil/natural gas drilling and/or oil/natural gas production. The locking device comprises a sleeve, which is axially displaceable relative to the piston and which is threadedly engaged with a rotatably supported rod for displacement in an axial direction. The rod is drivingly connected to a motor. In order to be able to determine, by means of simple structural measures, the position of the piston for each of its positions in a sufficiently precise and reproducible manner, the locking device is provided with a position sensor, which extends from a fixed point through a longitudinal bore of the lock rod up to the hydraulic piston and by means of which a distance between the piston and the fixed point can be detected.