Frequency characteristics of a plastic optical fiber (POF) measured by two different methods are compared. One is the more conventional method in which network analyzer is used for direct measurements. The other is the method in which an impulse response waveform of the POF is transformed into frequency domain by Fourier Transform. The latter gives different results depending on how the influence of the measuring system is corrected. However, both methods give almost the same results when the correction is implemented by using the frequency characteristics obtained by the Fourier Transform of the impulse response waveform of the measuring system.