Controlled release particles are obtained by dissolving a hydrophobic antibiotic compound in a hydrophobic polymerizable vinyl monomer without the presence of a solvent, thereby preparing a hydrophobic solution, the hydrophobic antibiotic compound having a melting point of 100° C. or less, a polar term δp,compound of 2 to 8 [(J/cm3)1/2] of a solubility parameter (δ) and a hydrogen bonding term δh,compound of 5.5 to 9.5 [(J/cm3)1/2] of the solubility parameter (δ), the solubility parameter (δ) defined by Hansen and calculated by van Klevelen and Hoftyzer method; dispersing the hydrophobic solution in water; and polymerizing the polymerizable vinyl monomer in the presence of an oil-soluble initiator by radical polymerization, thereby producing a polymer having a polar term δp,polymer of 5 to 7 [(J/cm3)1/2] and a hydrogen bonding term δh,polymer of 8 to 10 [(J/cm3)1/2] of the solubility parameter (δ).