The chemical state conversion of Ni species supported on SiO2 under CO and NO atmosphere was observed by in situ XAFS techniques, and the reversible and quantitative chemical state conversions between NiO and Ni(0) were clarified during the temperature-programmed reduction of NiO by CO and the temperature-programmed oxidation of Ni(0) by NO. The evaluated chemical state of the SiO2-supported Ni species indicated the catalytic CO–NO reaction conditions using the Ni/SiO2 catalyst. The actual catalytic experiments demonstrated the achievement of the catalyzed CO–NO reaction at 873 K. The dynamic chemical state analysis by means of the time-resolved dispersive XAFS method revealed the existence of a transient Ni species, which was partially oxidized by the faster reaction with NO. The formed NiO species can oxidize the CO molecule, and thus the CO–NO reaction is driven by the redox conversion of the supported Ni species.