A method for production of a fusion protein in which an anti-transferrin receptor (anti-TfR) antibody is fused with a human iduronate-2-sulfatase (human I2S), comprising; (a) a step for culturing mammalian cells producing the fusion protein in a serum-free medium to let them secrete the fusion protein in the culture medium, (b) a step for collecting culture supernatant by removing the mammalian cells from the culture medium, and (c) a step for purifying the fusion protein from the culture supernatant, characterized in that the fusion protein is purified by a column chromatography employing as a solid phase a material to which a substance having affinity for the fusion protein has been bound, wherein the substance having affinity for the fusion protein is Protein A, a column chromatography employing as a solid phase a material having affinity for the phosphate group, wherein the material having affinity for the phosphate group is hydroxyapatite, and a size exclusion column chromatography, in this order, wherein the human I2S: (a) has at least 80% amino acid sequence identity to the amino acid sequence set forth as SEQ ID NO:5, and has an activity as human I2S; (b) has at least 90% amino acid sequence identity to the amino acid sequence set forth as SEQ ID NO:5, and has an activity as human I2S; (c) has the amino acid sequence set forth as SEQ ID NO:5 with 1 to 10 amino acid substitutions, deletions or additions introduced, and has an activity as human I2S; (d) has the amino acid sequence set forth as SEQ ID NO:5 with 1 to 5 amino acid substitutions, deletions or additions introduced, and has an activity as human I2S; or (e) has the amino acid sequence set forth as SEQ ID NO:5 with 1 to 3 amino acid substitutions, deletions or additions introduced, and has an activity as human I2S. (a) a step for culturing mammalian cells producing the fusion protein in a serum-free medium to let them secrete the fusion protein in the culture medium, (b) a step for collecting culture supernatant by removing the mammalian cells from the culture medium, and (c) a step for purifying the fusion protein from the culture supernatant, characterized in that the fusion protein is purified by a column chromatography employing as a solid phase a material to which a substance having affinity for the fusion protein has been bound, wherein the substance having affinity for the fusion protein is Protein A, a column chromatography employing as a solid phase a material having affinity for the phosphate group, wherein the material having affinity for the phosphate group is hydroxyapatite, and a size exclusion column chromatography, in this order, (a) has at least 80% amino acid sequence identity to the amino acid sequence set forth as SEQ ID NO:5, and has an activity as human I2S; (b) has at least 90% amino acid sequence identity to the amino acid sequence set forth as SEQ ID NO:5, and has an activity as human I2S; (c) has the amino acid sequence set forth as SEQ ID NO:5 with 1 to 10 amino acid substitutions, deletions or additions introduced, and has an activity as human I2S; (d) has the amino acid sequence set forth as SEQ ID NO:5 with 1 to 5 amino acid substitutions, deletions or additions introduced, and has an activity as human I2S; or (e) has the amino acid sequence set forth as SEQ ID NO:5 with 1 to 3 amino acid substitutions, deletions or additions introduced, and has an activity as human I2S.