PURPOSE: To obtain a catalyst particle hardly lowered in catalytic activity and usable for a long time by providing a core material composed of a resin particle specified in its particle size, the metal oxide layer formed on the surface of the core material and the metal layer with a specific thickness formed on the metal oxide layer.

CONSTITUTION: A catalyst particle is constituted of a resin particle with a particle size of 0.05-5mm being a core material, the metal oxide layer 2 formed on the surface of the core material 1 and the metal layer 3 with a thickness of 0.01-5μm formed on the surface of the metal oxide layer 2. At that time, the wt. ratio of the resin particle and the metal oxide layer is set to the range of 99.9:0.1-90:10. A metal component selected from a group consisting of palladium, platinum, nickel, copper and an alloy based on at least one of those metals is added to the metal layer. This catalyst particle is used in the reaction in a gas-liquid mixed phase system, that is, oxidation reaction in water treatment, the reforming of petroleum for a fuel battery and the oxidation treatment reaction of dissolved gas.