The accelerator-driven system (ADS) is a promising potential method to achieve the nuclear transmutation of radioactive waste (minor actinides and long-lived fission products) from nuclear power plants. When applying ADS to nuclear transmutation, spallation neutrons (highenergy neutrons) obtained by an injection of high-energy protons onto a heavy metal target are employed. Among the basic research projects into ADS, at the Kyoto University Critical Assembly (KUCA), a series of reactor physics experiments coupled with the fixed-field alternating gradient (FFAG) accelerator is being carried out, and the neutron characteristics of ADS are investigated through static and kinetic analyses by means of experiments and calculations.