A group of metal magnetic nanoparticles is provided. The metal magnetic nanoparticle includes a core having a noble metal cluster of a diameter of 3 nm or less; and a metal shell, formed to surround the core, having noble metal atoms randomly distributed therein; wherein the metal shell has a noble metal atom content: (number of noble metal atoms)/(number of whole metal atoms).times.100 of 1 to 15 at. %.