The spectra of bremsstrahlung from very thick lead targets bombarded by 15-, 20-, and 25-MeV electrons from a linear accelerator were measured with activation detectors. This activation method has the advantage of being available for measuring the spectrum of a bremsstrahlung burst and being suitable for determining the distribution of the photon energy spectrum in the medium. By use of the most accurate photonuclear cross-section data available, the bremsstrahlung spectra were evaluated with the LYRA unfolding code. The evaluated spectra were in good agreement with the calculated spectra, especially at 9 deg for 15- and 20-MeV electrons. It is concluded that the spectrometry of bremsstrahlung radiation by activation detectors can satisfactorily represent the variation of bremsstrahlung spectrum as a function of incident electron energy. The spectra obtained by this method are very much dependent on the photo-nuclear cross-section data of activation detectors. This activation experiment can be used as an integral experiment to evaluate photonuclear cross-section data by coupling with the reference calculations.