Provided is a highly uniform sialylated sugar chain added polypeptide having interferon β activity.The said polypeptide is any one polypeptide selected from the group consisting of the following (1) to (4): (1) a polypeptide comprising an amino sequence expressed by sequence No.1, (2) a polypeptide comprising an amino sequence expressed by sequence No.1, in which one or several amino acids being deleted, substituted or added, (3) a polypeptide being an analog of interferon β, and (4) a polypeptide having 80% or higher homology to the polypeptide comprising an amino sequence expressed by sequence No.1, in which, four to six amino acids are substituted by a sugar chain added amino acid, and the non-reducing end of the sugar chain being all sialylated.本發明之課題係提供附加有高均一性之唾液酸化糖鏈且具有干擾素β活性的多肽。該多肽係在選自下述(1)至(4)：(1)由序列編號1所示之胺基酸序列構成之多肽，(2)在由序列編號1所示之胺基酸序列構成之多肽中，刪除、置換或附加1或數個胺基酸而成之多肽，(3)為干擾素β之類似物的多肽，以及(4)對比於由序列編號1所示之胺基酸序列構成之多肽，具有80%以上相同性的多肽所構成之組群之任一多肽中，4至6處之胺基酸以附加糖鏈之胺基酸置換，且該糖鏈之非還原末端皆被唾液酸化之附加糖鏈之多肽。
