PURPOSE: To avoid the generation of a permanently folded part on a magnetic tape, through the constitution that the lead-off of the tape is temporarily stopped when the tip of the magnetic tape passes through an air blowout hole and is led out by the prescribed length, and restarting threading afterward.

CONSTITUTION: A magnetic tape is led off by rotating a file reel 1 and the rotation of the file reel 1 is stopped when the tip passes through a sensor consisting of a light emitting element 10 and a photodetector 11 and led off by the prescribed length, thereby stoping the lead-off of the magnetic tape tentatively. Thus, the tip of the magnetic tape is drawn up to a tape guide surface 7, with a negative pressure formed by an air flow for threading blown out from the 1st air blowout hole 8, the rotation of the file reel 1 is again restarted for threading. Even if the threading is failed, the excessive tape further led off is contained in a vacuum column 4 with the air flow blown out of the 2nd air blowout hole 9, as shown in broken lines.