A tensioner for endless driving elements, such as chains and belts, comprising a tensioning piston provided with locking means, and a locking piston which is adapted to be engaged with and disengaged from the locking means so as to arrest or release the movement of tensioning piston, the locking piston comprising an operating section causing locking piston to move to the arresting position and to the release position, and a locking area used for engagement with the locking means of the tensioning piston and arranged separately from the operating section.