A tensioning apparatus for a circulating element, such as a belt or a band, has a basic body in which a first chamber is formed. A piston is positioned in the first chamber and carries a piston rod. The piston can be displaced in the first chamber by the use of a pressure fluid. Either the piston rod or the base body is attached to the carrier for the belt or band. Either the piston rod or the basic body carries a head piece which can be moved in contact with the circulating element. A locking mechanism for the piston rod can be engaged and disengaged by the use of fluid pressure.