PURPOSE: To industrially and advantageously obtain the subject oligosaccharide useful for medicines, functional foods, reagents for research, etc., by extracting an oligosaccharide containing sialic acids from a defatted egg yolk of an avian egg with water or a salt solution, then ultrafiltering the resultant extract or further enzymically treating the prepared ultrafiltrate.

CONSTITUTION: Water or a salt solution is added to a defatted egg yolk of an avian egg to stir the resultant mixture at 4-80°C, preferably 4-40°C for 10min to 3 days. The resultant mixture is then filtered to separate insoluble proteins and one or more endo type proteases such as actinase E, chymotrypsin, trypsin or thermolysin are added to the extract filtrate to digest the resultant mixture at 50°C for 16hr. The enzymically digested liquid is ultrafiltered and the digested liquid is then desalted with a reverse osmosis membrane and freeze-dried. Thereby, the objective oligosaccharide, containing the sialic acids and useful as a medicinal material, a functional food material and a regent for scientific research is simply obtained on an industrial scale at a low cost.