the optical fiber was repeatedly produced with varying z which is the distance from the shrinking part of the preform 1 to the outer diameter measuring device during the production the diameter of the optical fiber was measured by the measuring device and the diameter of the obtained optical fiber of which coating was stripped that is a true diameter of the optical fiber was actually measured by a precise micrometer thus it is found that in the case of the drawing rate of 300 m min the difference between the measured diameter of the optical fiber with the measuring device and the true diameter of the optical fiber is less than 0 5 when the optical fiber is cooled to below a temperature at which the shrinkage of the optical fiber diameter under tension at the point where the outer diameter is measured with the measuring device is 0 5 or less thus the position at which the measuring device is disposed is determined on the basis of the estimation of the fiber temperature according to the equation i and the several experiments as follows firstly the difference between the measured outer diameter and the true one is obtained with varying the position of the measuring device then a relation between the difference and the measuring position is established finally the position is determined at which the difference.