A light-diffusing-member manufacturing method includes a step of forming a light diffusion portion on one surface side of a base, by developing an exposed negative photosensitive resin layer with an alkali developing solution, and a step of performing an acid treatment on the light diffusion portion with an acid solution, after removing the alkali developing solution which is adhered to the light diffusion portion and suspended matter in the alkali developing solution in the negative photosensitive resin layer, so as to lower an ionization degree of the negative photosensitive resin layer which is in an ionized state due to the alkali developing solution.