The present invention provides a Scandium catalyst that can be used in water or water-soluble organic solvent with no leaching of Scandium. Provided is a gold-polymer nanostructure-immobilized Scandium catalyst, which is formed by preparing, in liquid phase, a mixture comprising gold clusters with from 1 to 50 nm of diameter, disulfide monomer, sulfonic acid salt of disulfide and Lewis acid metal compound represented by ScY3, wherein Y is OSO2CF3 etc., and polymerizing the mixture in the presence of a radical polymerization initiator, wherein the disulfide monomer is represented by the formula below: CH2═CH—R1—S—S—R1—CH═CH2 wherein R1 represents a divalent hydrocarbon, which may contain an ether bond, and the sulfonic acid salt of disulfide is represented by the formula below: MO3S—R2—S—S—R2—SO3M wherein R2 represents a divalent hydrocarbon, which may contain an ether bond, and M represents an alkali metal. This catalyst is useful as a catalyst for aldol reactions, cyanolation reactions, allylation reactions, Michael reactions, Mannich reactions, Diels Alder reactions and Friedel Crafts reactions in water or water-soluble organic solvent.