A method for manufacturing ultrafine fibers having an average diameter of less than 1 μm is implemented by an apparatus including a feeder and a drawing chamber in communication with the feeder via an orifice having a pressure difference. The method includes introducing a multifilament to the drawing chamber under the condition that the ratio of the cross-section of the multifilament to the cross-section of the orifice rectifier is 50% or less, and irradiating the discharged multifilament such that the center of the multifilament melted thereby is located 1 to 15 mm apart vertically below the orifice outlet to melt the leading portion of the multifilament and cause the multifilament to swing at a maximum angle of 5 to 80 degrees to the central orifice axis within a conical space, such that the melted leading portion of the multifilament is drawn by an air stream generated by the pressure difference.