To provide an apparatus whereby a uniform glass fiber with a smaller diameter can be manufactured.

Above a hollow rotor 1, an annular flame-ejecting nozzle 2 is arranged concentrically with the hollow rotor 1. Incomplete combustion gas is ejected from the flame-ejecting nozzle 2 and burned in an adjusting zone 5 located at radially outer side of the hollow rotor 1 to heat the adjusting zone 5 to a temperature equal to or above the softening point of glass. Between the hollow rotor 1 and the flame-ejecting nozzle 2, an annular blaster 3 is arranged concentrically with the hollow rotor 1. A nozzle hole 31 spouts air which is cooler than the softening point of glass onto thick-fiber glass 4 passing through the adjusting zone 5. The thick-fiber glass 4 is drawn into small-diameter fibrous glass 41 by an aerodynamic friction between the thick-fiber glass 4 and the air ejected from the nozzle hole 31.