To provide a catalyst composition which can obtain excellent reaction efficiency and cost reduction with respect to automobile exhaust gas and the like in comparison with conventional exhaust gas purifying catalysts.

The catalyst composition 100 comprises a metal oxide 102 and an oxygen source 108, and has oscillating oxygen 106 oscillatable around a metal atom 104 of the metal oxide 102. Oxygen generated from the oxygen source 108 moves toward the metal oxide 102 to act as oscillating oxygen. The metal oxide 102 and the oxygen source 108 are selected so that adsorption free energy gain due to oxygen adsorption to the metal oxide 102 is larger than adsorption free energy gain due to oxygen adsorption only to the oxygen source 108.