PURPOSE: To produce a flat film without trimming loss and unevenness by, while splitting a cylindrical bubble into a flat-shaped piece in its flowing direction with a cutting knife, moving the cutting knife at the right angle to the flowing direction of the bubble in a given period or according to the speed programmed, and then taking up the flat film.

CONSTITUTION: A cylindrical bubble 5 is split with a cutting knife 7, and then taken up by a nip roll 8. When splitting the cylindrical bubble 5, the cutting knife 7 is moved mechanically along a guide 9, and unevenness of the film is dispersed. At that time, the cutting knife 7 is moved in a given period or according to the programmed speed. The cutting knife 7 is moved almost at right angle to the flowing direction of the bubble. The moving range of the cutting knife 7 when splitting the cylincrical bubble 5 is different according to films to be prepared and cannot be decided, but usually the moving range should preferably by ±10 mm or more. By said arrangement, a flat film with dispersed unevenness is prepared.