Abstract: A method of measuring the tension of drawing an optical fiber by means of the oscillation waveform of the optical fiber being drawn according to the invention can accurately, reliably and easily detect a peak frequency in the frequency spectrum of an oscillation waveform without being affected by noise. With the method of measuring the tension of an optical fiber being drawn by determining the fundamental oscillation frequency of the optical fiber from a peak in the spectrum of the linear oscillation waveform of the optical fiber, the detection of a peak frequency in the frequency spectrum of a linear oscillation waveform comprises steps of conducting an initial detecting operation over a frequency range expected to contain a peak frequency and conducting subsequent detecting operations over respective frequency ranges obtained by sequentially aligning the median frequency with the peak frequency detected in the immediately preceding detecting operation.