Abstract: The stack forming device according to the invention comprises a traction surface which is rotatably driven. The portion of the trailing edge of a sheet to be stacked is kept in contact with the traction surface so that the trailing edge of the sheet is displaced in synchronism with said surface. This displacement is carried out over a period which follows the abutment of the leading edge of the sheet against a stop provided on a stacking platform. As a consequence the sheet acquires a convex configuration and is finally propelled against the platform or the preceding sheet after release by the traction surface. The device may be equipped with supplementary blowers in order to facilitate the release from the traction surface. It is of particular interest for high speed stacking of photographic film sheets, preferably double-side coated ones.