PROBLEM TO BE SOLVED: To provide a method for producing nanoparticles composed of an amphiphilic block polymer with a uniform particle diameter.SOLUTION: In a method for producing nanoparticles composed of an amphiphilic block polymer having a hydrophilic block and a hydrophobic block, a laminar flow of a polymer solution is formed by introducing a solution including the amphiphilic block polymer having the hydrophilic block and the hydrophobic block in an organic solvent into a polymer solution supply channel, at least two laminar flows of a water-based liquid are formed by introducing the water-based liquid into at least two water-based liquid supply channels, and the nanoparticles composed of the amphiphilic block polymer are formed by merging the laminar flow of the polymer solution with the two laminar flows of the water-based liquid such that the laminar flow of the polymer solution is sandwiched between the mentioned at least two laminar flows of the water-based liquid.
