Polymer particles are used in various consumer products, such as cosmetics and paints. However, large amounts of surfactants are used to synthesize polymer nanoparticles, which increase the environmental load. In this study, we propose a method for synthesizing polymer nanoparticles without using surfactants. In this method, the nanoparticles are polymerized inside the melamine foam (MF) used as a reaction field. A study of the distribution of particles in each area of the foam revealed that nanoparticles could be obtained with a relatively uniform size in the height direction. We successfully controlled the particle size by controlling the porosity of the MF by filling it with spherical glass beads of 50 mu m diameter. As no surfactant is used, this polymerization method has a low environmental impact. Because the surface of the polymer nanoparticles is not contaminated by surfactants, they can be used in medical applications. Our study provides a new direction in the synthesis and application of polymer nanoparticles.