It was found that Pt particles on Al2O3-supported Pt catalysts prepared using Pt complex nanoparticles formed in a water-in-oil microemulsion became very small and uniform compared to those prepared using reduced Pt metal nanoparticles or by an impregnation method. Moreover, the catalytic activity of the catalyst composed of very small Pt particles, which was prepared using the complex nanoparticles, was higher in the NO-CO reaction than those of the other catalysts.