An obj ect of the present invention is to provide a dispersion of noble metal-supported photocatalyst particles, which exhibits high photocatalytic activity, and also has stable dispersibility that enables prevention of precipitation of photocatalyst particles in a dispersion medium a method for producing the same a hydrophilizing agent and a photocatalytic functional product. Disclosed is a method for producing a dispersion of noble metal-supported photocatalyst particles wherein noble metal-supported photocatalyst particles including a noble metal supported on a surface of photocatalyst particles are dispersed in a dispersion medium, the method including the steps of (1) adjusting the pH of a raw dispersion in which the photocatalyst particles are dispersed in the dispersion medium of the raw dispersion and a precursor of the noble metal is dissolved in the raw dispersion, in a range from 2.8 to 5.5, and also adjusting the amount of oxygen dissolved in the raw dispersion to 1.0 mg/L or less (2) irradiating the raw dispersion with light having energy larger than or equal to that of a bandgap of the photocatalyst particles and (3) adding a sacrificial agent to the raw dispersion after the step (2), and also irradiating the raw dispersion with light having energy larger than or equal to that of a bandgap of the photocatalyst particles, thereby supporting the noble metal on a surface of the photocatalyst particles.
