A composition which has high 4'-GL purity and can be used as a reference standard for various analyses can be obtained by a more convenient method than one conventionally used. A method for preparing a high-purity 4'-GL composition includes the steps of: (A) subjecting a 4'-GL-containing galacto-oligosaccharide to activated carbon column chromatography, and performing stepwise elution with plural organic solvent aqueous solutions, wherein the organic solvent aqueous solutions are used such that the concentration of the organic solvent in the organic solvent aqueous solution is higher than the concentration of the organic solvent in the immediately preceding organic solvent aqueous solution with respect to a series of elutions； and (B) adding an organic solvent to the final fraction eluted in step (A), and crystallizing the 4'-GL.
