To readily and inexpensively provide an acidic oligosaccharide having a high content of uronic acid in mass.

The method for producing the acidic oligosaccharide having the uronic acid residue in the molecule includes extracting a corn fiber and/or a corncob with hot water at 80-150C in slurry concentration of 2-30 mass%, and subjecting the extract to ion-exchange treatment. In the production method, the pH of the slurry of the corn fiber and/or corncob is preferably acidic of 1.0-4.0, or basic of 9-13, and a decomposition treatment with a hemicellulase can be carried out before or after the hot water extraction treatment. Preferably, an enzyme treatment with a glucide-splitting enzyme or a cellulase is carried out before the hot water extraction or hemicellulase treatment.