A bag filling plant includes a detection device for detecting a thickened section of a film web associated with an end of a first film and a start of a second film. The detection device includes a first film roll and a second film roll, which contact the film web on side areas aligned opposite each other and which form a conveyer gap for the film web, with the first film roll being movably supported relative to the second film roll in order to alter the conveyer gap. The detection device includes a sensor device that detects a motion of the first conveyer roll.