Hyaluronic acid oligosaccharides having a size selected from sizes of 4 to 60 saccharides, fractions containing the hyaluronic acid oligosaccharides and having particular physicochemical properties, and drugs containing the same. The hyaluronic acid oligosaccharides of the present invention are extremely useful, because they exert superior pharmaceutical effects as active ingredients of a heat shock protein expression promoter, cell death inhibitor, cell injury inhibitor and cell and tissue protecting agent (e.g., organ preservation agent, antiulcer agent, antihepatopathic agent, IL-10 production promoter or IL-8 production inhibitor) and exhibit high safety.