An oxide sintered body which, when made into an oxide semiconductor thin film by sputtering, can achieve low carrier density and high carrier mobility, and a sputtering target using said oxide sintered body are provided. This oxide sintered body contains indium, gallium and zinc as oxides. The gallium content is 0.08 or more and less than 0.20 in terms of Ga/(In+Ga) atomic ratio, and the zinc content is 0.0001 or more and less than 0.08 in terms of Zn/(In+Ga+Zn) atomic ratio. This crystalline oxide semiconductor thin film is formed with the oxide sintered body as a sputtering target, and can achieve a carrier density of 8.0×1017 cm−3 or less and a carrier mobility of 10 cm2/V·s or greater.