This paper describes the formation of cuprous oxide (Cu2O) particles similar to 10 nm in diameter and their reduction by hydrogen gas in porous glasses. Nanoparticles of Cu2O coated with metallic copper are expected to show high third-order optical nonlinear susceptibilities (X-(3)). The porous glass is a medium in which nanoparticles can be partially reduced without forming agglomerations. Images obtained using transmission electron microscopy showed that particles with the desired core-shell structure were actually formed, even though some particles were not reduced uniformly from the surface. The X-(3)/alpha (alpha: absorbance) values in relevant conditions (10(-13) esu . cm) were similar to that of copper because of the formation of a byproduct of copper nanoparticles on reduction.