Abstract: A method of helically cutting a tubular film of thermoplastic material, in which a film in lay flat form, discharged from a reel, is brought to rotate around the middle axis of the film by means of rotating unwind devices, and while the rotating tube proceeds over the mandrel it is cut to non-tubular form by a knife, the position of which is fixed in relation to the surroundings, and the cut film is taken off from the mandrel, characterized in that the inflated film while being forwarded towards the mandrel or while it passes the upstream end of the mandrel, or both, is supported by driven support means placed around the outside of the tubular film, the movement of the support means being adapted to fit with the combined rotation and forwarding movement of the film.