Controlled release particles are obtained by dissolving a hydrophobic antibiotic compound with a hydrophobic polymerizable vinyl monomer to prepare a hydrophobic solution, blending water with an emulsifier to prepare an aqueous emulsifier solution, emulsifying the hydrophobic solution in the aqueous emulsifier solution, and polymerizing the polymerizable vinyl monomer by mini-emulsion polymerization in the presence of a polymerization initiator, thereby producing a polymer containing an antibiotic compound and having an average particle size of below 1 μm.