The present invention relates to a method comprising diluting aqueous solution A, which contains particles Z comprising self-organizing molecules as a particle-constituting ingredient and which contains a water-miscible organic solvent in a concentration of 20 vol% or higher, with aqueous solution B1 to thereby obtain aqueous solution C, which contains the particles Z having a number-average particle diameter of 200 nm or smaller. For the dilution is used a channel structure for dilution which comprises a D1 introduction channel and a D2 introduction channel that have been disposed on the upstream side, a dilution channel disposed downstream from a confluence of the D1 introduction channel and the D2 introduction channel, and one or more Dn introduction channels provided somewhere to the dilution channel. The dilution comprises supplying aqueous solution A and aqueous solution B1 respectively to the D1 introduction channel and the D2 introduction channel, adding one or more aqueous solutions Bn through the Dn introduction channels to the aqueous solutions that are running through the dilution channel, and obtaining aqueous solution C, which contains the water-miscible organic solvent having been diluted to or below a concentration at which the particles can be stably present, through the outlet of the dilution channel. The present invention provides: a method for producing a nanoparticle-containing aqueous solution in which the particle diameters of particles comprising self-organizing molecules as a particle-constituting ingredient can be maintained as much as possible； and a device for use in the production method.La présente invention concerne un procédé comprenant la dilution d'une solution aqueuse A, qui contient des particules Z comprenant des molécules auto-organisatrices en tant qu'ingrédient constituant des particules et qui contient un solvant organique miscible dans l'eau à une concentration de 20 % en volume ou plus, avec une solution aqueuse B1 pour obt
