Crystal growth under hydrothermal treatment at 150 degrees C was observed for the pre-calcined TiO2-SnO2 precursor obtained by the calcination at 250 degrees C. This phenomenon was observed for the trivalent titanium ion (Ti3+) used as the starting material for hydrolysis. The peculiar crystal growth phenomenon was thought to be originated from the oxidation of titanium from trivalent to tetravalent, because such peculiar crystal growth under hydro thermal conditions was not observed when tetravalent titanium was used.