A nanoparticle-containing solution comprising nanoparticles of a metal oxide, and a solution obtained by dissolving, in an organic solvent, a phosphoric acid ester and a reactive group-containing carbonyl compound as hydrophobic treatment agents, the phosphoric acid ester having an alkylene oxide chain, and an alkyl group or allyl group at an end, and the reactive group-containing carbonyl compound having at least a vinyl group and a carboxyl group or cyclic ester group, and having a solubility parameter calculated by Fedors' method of 10.0 to 12.5.