PURPOSE: To open weak fibers or short cotton fibers without damaging or peeling fibers by making the fiber opening end edges of blades flat, bringing it into contact with fibers forming a thread end, and performing a weak beating and opening action.

CONSTITUTION: When a thread end to be opened is sucked into a fiber pipe 2 through a thread end suction opening by the sucking action of a suction source, the thread end brought into contact with the fiber opening end edges 7 of the blades 6 of a beater 4 rotated by the suction air stream is unraveled by the beating action of the beater 4, and fibers constituting the thread end are aligned in a parallel state. The fiber opening end edges 7 of blades 6 have a flat face and are brought into contact with fibers constituting the thread end at the uniform and short length and perform a soft beating action as compared with blades 6 having saw-toothed irregularities on fiber opening end edges 7. The fiber opening action beating weak fibers such as wool and rayon or short fibers such as cotton is performed without damaging fibers.