Provided is a sputtering target of sintered Ti—Nb based oxide, wherein the sputtering target consists of titanium (Ti), niobium (Nb), and remainder being oxygen and unavoidable impurities, and the atomic ratio of Ti and Nb is 0.39≦(Nb/(Ti+Nb))≦0.79. The sputtering target of sintered Ti—Nb based oxide has a high refractive index and a low extinction coefficient. Also provided is a thin film of Ti—Nb based oxide obtained by using the foregoing target, which enables high-rate deposition. The thin film has superior transmittance, is subject to minimal reduction and variation of reflectivity, and is useful as an interference film or a protective film of an optical information recording medium, or as a part of a constituent layer of an optical recording medium. The thin film can also be applied to a glass substrate; that is, it can be used as a heat reflecting film, an antireflection film, or an interference filter.