In one embodiment of the present application, a polypeptide capable of binding to a sugar chain is disclosed, particularly a high-mannose-type sugar chain bound to an antibody, more preferably a sugar chain bound to a chicken antibody. Also disclosed is a method for the purification of an antibody (specifically a chicken antibody) as a representative application of the polypeptide. Further disclosed is means for the purification. The polypeptide, BML-17, is a novel lectin made of 168 amino acid residues isolated from Bryopsis maxima. By using BML-17, it becomes possible to purify an antibody (e.g., a chicken antibody) readily and with high efficiency.