To provide a catalyst that has sufficient catalytic activity as a platinum catalyst and yet is readily separated from a target material and can be reused. A resin-supported platinum cluster catalyst containing a resin layer comprising a platinum particle core, the average diameter of which is from 1 nm to 10 nm, and a compound containing nitrogen (general formula R1NH2) surrounding the core. This catalyst can be obtained by forming a platinum complex from a compound containing nitrogen represented by the general formula R1NR22 (in the formula, R1 represents a carrier with a hydrophilic molecular chain and R2 represents a hydrogen atom, an alkyl group and the like) and a platinum compound represented by the general formula M(PtX4-nR3n)m (in the formula, M represents an alkali metal and the like, X represents a halogen atom and the like, R3 represents an unsaturated organic ligand, n represents an integer from zero to four and m represents an integer indicating the electrical charge of the metal, M) and allowing this platinum complex to react with a reducing agent in an aqueous solvent.