A keratan sulfate oligosaccharide which comprises from two to five sugar units and has sulfated N-acetylglucosamine at the reducing end and in a molecule of which at least two hydroxyl groups are sulfated, preferably, which contains at least disaccharide represented by the formula Gal(6S)-GlcNAc(6S) (in the formula, Gal, GlcN, Ac, and 6S represent a galactose, a glucosamine, an acetyl group, and a 6-O-sulfate ester, respectively) as a constitutional ingredient, and/or pharmaceutically acceptable salt thereof are used as active ingredients of anti-inflammatory agents, antiallergic agents, immunomodulators, cell differentiation inducers, and apoptosis inducers.