A method for preparing biodegradable microspheres comprising: (a) forming a biodegradable polymer solution by dissolving a biodegradable polymer alone; or a biodegradable polymer and a drug in an organic solvent; (b) uniformly mixing the biodegradable polymer solution prepared in the step (a) with an aqueous solution containing a surfactant, to form an emulsion containing the biodegradable polymer solution as a dispersed phase and an aqueous solution containing the surfactant as a continuous phase; (c) extracting and evaporating the organic solvent from the dispersed phase in the emulsion of step (b) to the continuous phase to produce microspheres, wherein a part of the continuous phase containing the extracted organic solvent is removed, while a new continuous phase is supplied; and (d) recovering the microspheres from the continuous phase containing the produced microspheres of step (c), wherein the part of the continuous phase in step (c) is removed discontinuously or continuously, and (i) when discontinuously removing the part of the continuous phase in step (c), the removed continuous phase is a volume ratio of 40 % (v/v) to 99 % (v/v) based on the total volume of the continuous phase, or is a weight ratio of a remaining continuous phase except for the one time weight ratio of continuous phase of the microparticle in the step (c), or (ii) when continuously removing the part of the continuous phase in step (c), the rate of removal of the continuous phase is in the range of 2 % (v/v) to 200 % (v/v)/min based on the total volume of the continuous phase. (a) forming a biodegradable polymer solution by dissolving a biodegradable polymer alone; or a biodegradable polymer and a drug in an organic solvent; (b) uniformly mixing the biodegradable polymer solution prepared in the step (a) with an aqueous solution containing a surfactant, to form an emulsion containing the biodegradable polymer solution as a dispersed phase and an aqueous solution containing the surfactant as a continuous phase; (c) extracting and evaporating the organic solvent from the dispersed phase in the emulsion of step (b) to the continuous phase to produce microspheres, wherein a part of the continuous phase containing the extracted organic solvent is removed, while a new continuous phase is supplied; and (d) recovering the microspheres from the continuous phase containing the produced microspheres of step (c), wherein the part of the continuous phase in step (c) is removed discontinuously or continuously, and (i) when discontinuously removing the part of the continuous phase in step (c), the removed continuous phase is a volume ratio of 40 % (v/v) to 99 % (v/v) based on the total volume of the continuous phase, or is a weight ratio of a remaining continuous phase except for the one time weight ratio of continuous phase of the microparticle in the step (c), or (ii) when continuously removing the part of the continuous phase in step (c), the rate of removal of the continuous phase is in the range of 2 % (v/v) to 200 % (v/v)/min based on the total volume of the continuous phase. (i) when discontinuously removing the part of the continuous phase in step (c), the removed continuous phase is a volume ratio of 40 % (v/v) to 99 % (v/v) based on the total volume of the continuous phase, or is a weight ratio of a remaining continuous phase except for the one time weight ratio of continuous phase of the microparticle in the step (c), or (ii) when continuously removing the part of the continuous phase in step (c), the rate of removal of the continuous phase is in the range of 2 % (v/v) to 200 % (v/v)/min based on the total volume of the continuous phase.