Method for producing metal oxide nanoparticles, and metal oxide nanoparticles

A method for producing metal oxide nanoparticles, which comprises: a step wherein an inorganic oxide carrier and a metal salt are mixed in a dispersion medium that contains water, thereby having the surface of the inorganic oxide carrier support metal ions that constitute the metal salt (first step); a step wherein the dispersion liquid obtained in the first step and a basic substance are mixed, thereby converting the metal ions into a metal hydroxide (second step); and a step wherein the metal hydroxide supported on the surface of the inorganic oxide carrier is subjected to a hydrothermal reaction together with the inorganic oxide carrier in the presence of water that is in a subcritical state or in a supercritical state (fourth step).