Results of deconvolution applied to actual data of the energy distribution function of electrons based upon the Druyvesteyn method are presented. The measurement is done by the beat method which is best of alternating current (AC) methods for obtaining the function. Among well-known numerical methods, the fast Fourier transformation method (FFT) is found to be most favorable in view of memory size, convergency, and calculation speed, if a suitable smoothing process is adopted. The basic principle, a circuit diagram and exper imental results are described.