Method for manufacturing ultrafine fiber

A method for manufacturing ultrafine fibers having an average fiber diameter of less than 1 μm with an apparatus including a feeder of an original filament and a drawing chamber in communication with the feeder via an orifice having an inlet and an outlet, the orifice having a pressure difference of 20 kPa or greater between the inlet and outlet by irradiating the original filament introduced from the orifice to the drawing chamber with a laser beam, the method comprising:

• introducing the original filament comprising a multifilament of a bundle of 10 or more monofilaments to the drawing chamber under such a condition that the ratio S2/S1 of the total cross section S2 of the multifilament to the cross section S1 of a rectifier of the orifice is 50% or less;
• and
• irradiating the multifilament discharged from the orifice with a laser beam such that the center portion of the multifilament melted thereby is located 1 mm to 15 mm apart vertically below the outlet of the orifice to melt the leading portion of the multifilament and to cause the entire multifilament to swing at random at a maximum angle of 5 degrees to 80 degrees to the central axis of the orifice within a conical space with the apex defined by the orifice port such that the melted leading portion of the multifilament is drawn by an air stream generated by the pressure difference.