Synthesizing polymer nanoparticles have required large amounts of surfactants, which increase environmental pollution. It is indispensable to develop a synthesis method for polymer nanoparticles that does not require surfactants. In the present study, a methylcellulose (MC) gel was used as a polymerization field to prepare polymer nanoparticles with a particle diameter less than 100 nm. MC gel　was a biocompatible hydrophilic polymer and low environmental load. This was used as a reactor to carry out the soap-free emulsion polymerization of styrene. The mesh width of the gel decreased with increasing MC concentrations. The three-dimensional network structure of the MC gel prevented the coagulation growth of the particles due to Brownian and thermal motions to decrease the size of the polystyrene particles. Polystyrene particles with the average size of 31 nm were obtained at an MC concentration of 1.0 wt%. In addition, curdlan and melamine foam, which had also three-dimensional network structures, were used as reaction fields of soap-free emulsion polymerization of styrene to make polystyrene particles nanosized.