Search for the end of the energy spectrum of primary cosmic rays

The experiments on the search for the end of the cosmic-ray energy spectrum with the particle detector array technique and the optical (Cerenkov and/or fluorescence) technique are summarized. In the highest observed energy region, the suppression of the flux above about 6 x 10(19) eV, which corresponds to the so-called 'Greisen-Zatsepin-Kuz'min (GZK) cutoff energy', is confirmed by the new results from the High-Resolution Fly's Eye (HiRes) and the Pierre Auger Observatory (Auger). Summarizing the spectra observed from 10(14) to 10(20) eV with world wide experiments, there is still room for further investigations on the energy scale and the flux of primary cosmic rays. In order to establish the energy scale, it is necessary not only to understand the optical technique further, but also to determine the primary species in the highest energy region. Also, it is important to make more detailed measurements on the energy spectrum in the 10(17)-10(18) eV region. To observe the super-GZK events and the anticipated recovery of the spectrum above the GZK cutoff, the search for the end of cosmicray energy spectrum will be continued even after a century since the discovery of cosmic rays in 1912.