The invention provide a method for producing a quantum crystal of a metal complex encapsulating plasmon metal quantum dots. The presnt quantum crystals are produced by a method charactgerized in that an aqueous solution of plasmon metal complex made from a ligand and a plasmon metal selected from the group consisting of gold, silver, copper, nickel, zinc, aluminium, and platinum, is prepared and brought into contact with a metal carrier made of a metal or a metal alloy having an electrode potential lower than that of the plasmon metal in the aqueous solution, such that the plasmon metal complex is preciptated as quantum crystals arranged on the metal carrier, resulting in having a plasmon enhancement effect. The metal complex crystals encapsulate metal quantum dots, so that an excellent adsorption capability of sample to be detected, as well as an excellent surface plasmon exitation electrofield enhancement effects, are obtained as a result of the quantum size effect.