PROBLEM TO BE SOLVED: To provide a process for preparing an oligosaccharide derivative from an oligosaccharide mixture. SOLUTION: The process for preparing the oligosaccharide derivative includes the steps of: (a) introducing a lipophilic group into oligosaccharides of the mixture to obtain a mixture of oligosaccharide derivatives; (b) treating the oligosaccharide derivative mixture by serotonin affinity column chromatography; (c) conducting normal phase chromatography with use of an amino column or amide column after the step (b); and (d) treating the resulting eluate with a glycosidase before the step (c). COPYRIGHT: (C)2012,JPO&INPIT