PURPOSE: To obtain in high yield the title oligosaccharide as a Lactobacillus bifidus-proliferative factor from laver by adding a mixed solution of acetic acid, acetic anhydride and sulfuric acid to the polysaccharide from the seaweed classified as Rhodophyta laver (Porphyra) to make an acetolysis followed by deacetylation.

CONSTITUTION: A mixed solution of acetic acid, acetic anhydride and sulfuric acid is added to the polysaccharide from the seaweed classified as Rhodophyta laver (Porphyra) to make an acetolysis followed by deacetylation, thus obtaining the objective oligosaccharide. It is preferable that the above-mentioned polysaccharide be prepared by the following process: 50-100 pts.wt. of water is added to 1 pt.wt. of the above-mentioned seaweed or toasted laver prepared by roasting said seaweed followed by hot water extraction at 50-90°C and then filtering the solid contents off, and ethanol is added to the resultant solution so as to be ≥80wt.% in the ethanol concentration to obtain the aimed polysaccharide as precipitate.