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The purpose of the research is to clarify the generation mechanism of precursor electrons ahead of a hypersonic shock wave in argon. The triple-probe measurements and radiation measurements are carried out using a hypersonic shock tube. Theoretical analysis is also carried out using a one-dimensional photoionization model. The triple-probe measurements show that the electron temperature is almost uniform ahead of the shock wave and ranges from 5000 to 11000 K. In contrast to the electron temperature, the electron density increases exponentially as approaching the shock wave and lies between 1015 and 1017 m-3. These results show that considerable electrons exist in the region ahead of the shock wave. From the radiation measurements, argon ions are found to emit radiation in the region ahead of the shock wave, showing that argon ions are generated from argon atoms by photoionization. The behavior of electron temperature and density is well accounted for by the one-dimensional photoionization model with an assumption of photoionization as the primary ionization process ahead of the shock wave. In conclusion, precursor electrons are mainly generated by photoionization, making use of the radiation energy behind the shock wave.