Disclosed is a method for producing ultrafine particles of a Prussian blue-type metal complex wherein (A) an aqueous solution containing an anionic metal cyano complex having a metal atom M1 as the central metal is mixed with an aqueous solution containing metal cations of a metal atom M2 for precipitating a crystal of a Prussian blue-type metal complex having the metal atom M1 and the metal atom M2; then (B) a solution obtained by dissolving a ligand L in a solvent is mixed with the crystal of the Prussian blue-type metal complex for forming a dispersion liquid of ultrafine particles of the Prussian blue-type metal complex; and the (C) the Prussian blue-type metal complex is separated from the dispersion liquid as nanometer-sized ultrafine particles.