First, an ammonium acetate solution and an acetate solution of a metal such as Cd or Zn are mixed, so as to form an ammonium complex of the metal. Next, an apoferritin solution, listeria apoferritin (listeria Dps) or their recombinant and thioacetic acid are added to the thus obtained reaction solution, which is maintained at a pH of from 4.0 to 9.0. By allowing the reaction solution to stand for 12 hours or more, a complex of a nanoparticle including CdS or ZnS and apoferritin or listeria Dps is produced.