TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Method: The method of preparing catalyst particle further comprises, prior to reducing at least the surfaces of the particles composed of the second oxide to a first oxide having oxygen defects, pre-reducing at least the particles composed of the second oxide within the dispersion of particles composed of the second oxide. In the method of preparing catalyst particle: by reducing at least the surfaces of the particles composed of the second oxide to a first oxide having oxygen defects, an intermediate layer containing the first oxide is formed at the surfaces of the particles composed of the second oxide; when at least the particles composed of the second oxide are pre-reduced, a reducing agent is additionally mixed into the mixture of the dispersion of reversed micelles containing particles composed of the second oxide with the dispersion of reversed micelles containing platinum ions, and the mixture is heated after adding an alcohol to the mixture following formation of the outermost layer on the first oxide; a sacrificial reagent is additionally mixed into the mixture of the dispersion of particles composed of the second oxide with the dispersion of platinum ions, following which the mixture is irradiated with light; and an alcohol is added to the light-irradiated mixture, following which the mixture is heated. Preferred Components: The inner particle has a center particle and an intermediate layer covering the center particle, where the center particle contains a second oxide i.e. free of oxygen defects and that includes an element common with an element other than oxygen included in the first oxide, and the intermediate layer contains the first oxide. The second oxide is an oxide that, by reducing a surface of the center particle, has generated oxygen defects in the first oxide. The first oxide includes an element consisting of titanium, tin, tantalum, niobium or silicon. The catalyst particle has an average particle size of 2-20 nm. The outermost layer has a degree of coverage of 70-100% with respect to the inner particle. The outermost layer is a layer of three or fewer atoms. The carbon support is composed of at least one carbon material consisting of acetylene black, furnace black, carbon black, activated carbon, mesophase carbon or graphite. The dispersion of particles composed of the second oxide is a dispersion of reversed micelles containing particles composed of the second oxide. The dispersion of platinum ions is a dispersion of reversed micelles containing platinum ions. The dispersion of reversed micelles containing particles composed of the second oxide is obtained by mixing an aqueous solution or aqueous dispersion of particles composed of the second oxide with an organic solvent solution of a surfactant. The dispersion of reversed micelles containing platinum ions is obtained by mixing an aqueous solution of the platinum ions with an organic solvent solution of a surfactant. The second oxide is an oxide consisting of titanium(IV) oxide, tin(IV) oxide (both preferred), tantalum(V) oxide, niobium(V) oxide and silicon dioxide. The second oxide has a photocatalytic activity.