An oligosaccharide derivative, at higher homogeneity than fucose polysaccharide and rhamnose polysaccharide and with greater antiulcer effects, which is represented by the following general formula: Y—OCH(CH2NHR)2 wherein, Y represents an oligofucose or oligorhamnose at a polymerization number of 2 to 20, where the hydroxyl groups may or may not be partially modified in the form of sulfate ester, and R represents phenyl group, a higher alkylphenyl group, a higher alkyl group or —(CH2)n—NHX wherein n is an integer of 1 to 10 and X represents a higher alkanoyl group or an alkylamino group.