The invention provides a method for producing nanofibers of an aliphatic polyester resin (a biodegradable resin) with higher productivity than heretofore achieved, for example, without the need of cumbersome steps such as drawing and with operability at normal temperature. The method for producing nanofibers according to the invention includes (1) an extrusion step of extruding an organic solvent solution (S) into filaments in an aqueous solution (W) including a surfactant (SF) and water, the organic solvent solution (S) including an aliphatic polyester resin (A0) or an oil-soluble low block copolymer (A) including a block derived from an aliphatic polyester resin (A1) and a block derived from a hydrophilic polymer (A2), and an organic solvent (S1); and (2) a spinning step of collecting the filaments of the organic solvent solution (S) extruded in the step (1) while simultaneously diffusing or extracting the organic solvent (S1) of the organic solvent solution (S) into the aqueous solution (W), thereby forming fibers including the aliphatic polyester resin (A0) or the oil-soluble low block copolymer (A).