An oxide sintered body comprising indium and gallium as an oxide, characterized in that an In2O3 phase with a bixbyite-type structure forms a major crystal phase, and a GaInO3 phase of a β-Ga2O3-type structure, or GaInO3 phase and a (Ga, In)2O3 phase is finely dispersed therein, as a crystal grain having an average particle diameter of equal to or smaller than 5 μm, and other than indium and gallium, further one or more kinds of additive components selected from tin or germanium are contained, and the content of gallium is from 2 to 30% by atom as atom number ratio of Ga/(In+Ga+Sn+Ge), and the content of the additive components is from 1 to 11% by atom as atom number ratio of (Sn+Ge)/(In+Ga+Sn+Ge).