5 is changed by means of a mass flow controller flow rate control apparatus 9 so that the difference between the tip position resulting from the averaging processing and the target position is 0 see fig2 the control during this time is set as pi control which uses a proportional component relative to the difference and an integral component for preventing an offset by setting the target position as an intermediate value between two adjacent discrete values the tip position and the flow rate of the raw material gas did not change largely although having undergone constant and minute fluctuations as in the first embodiment example when the target position deviated by only a small amount such as about 1 20 of the distance between two discrete values for example the similar result has been observed however particularly advantageous results have been obtained when the tip position lies within a range of the dividing point of 0 4 0 6 with respect to two adjacent discrete values when the tip position lies in the vicinity thereof the tip position is adjustable by only a small change in the flow rate of the raw material gas silica particles were deposited in a system similar to the first embodiment example while the target position of a soot core tip is substantially equalized to the discrete value the result was divided between a time period in which.