The present invention provides an oligosaccharide derivative of the following fomula(I) or its salt or its ester, which has an excellent activity and a safety, and is useful as a medicament for prophylactic or therapeutic treating diabetes (wherein, ring A is a cyclic ring etc., R1 and R2 are the same or differrent, each represents an alkyl group, a hydroxymethyl group, n is 1 or 2).