Disclosed is a method for producing a modified animal fiber, the method includes step 1 (31, 32) of pre-oxidizing a cystine bond (—S—S— bond) present in an epidermal cell of an animal fiber to bring the cystine bond into a low oxidation state, step 2 (33) of oxidizing with ozone the pre-oxidized —S—S— bond to bring the —S—S— bond into at least one high oxidation state selected from di-, tri-, and tetra-oxidation states, and step 3 (34) of reductively cleaving the —S—S— bond in a high oxidation state. The method imparts shrink resistance and pilling resistance to an animal fiber. In the step 2 (33) ozone is microdispersed in an aqueous solution comprising an anionic surfactant having a C8-24 alkyl group, and the animal fiber is contacted with the ozone. Accordingly, the present invention provides a method for efficiently producing in a short period of time an animal fiber having excellent shrink resistance that barely undergoes felting when washed in an aqueous system in shrink proofing an animal fiber using ozone.