Abstract: In a process for manufacturing sacks of sheet material, flat tubes are fed one by one in the direction perpendicular to the discharging direction thereof, while the tubes are overlapped or shingled as offset by a predetermined distance. The tubes are wound one by one on a reel core with the help of a tape, which is connected at one end thereof to the reel core and continuously supplied with the rotation of the reel core. The tubes are secured on the reel core outside pressure supplied by the tape to form a tube wound reel having a desired diameter. After a certain time period and immediately before the susequent bottom-closing process, the tubes are unwound from the reel core by pulling the tape to rotate the reel. The overlapped tubes are then separated from each other and supplied one by one to a bottom-closing station.