An oxide sintered body is obtained by sintering indium oxide, gallium oxide and tin oxide. The oxide sintered body has a relative density of 90% or more and an average grain size of 10 μm or less. In the oxide sintered body, the relations 30 atomic %≤[In]≤50 atomic %, 20 atomic %≤[Ga]≤30 atomic % and 25 atomic %≤[Sn]≤45 atomic % are satisfied. [In], [Ga] and [Sn] are ratios of contents (atomic %) of indium gallium and tin, respectively, to all metal elements contained in the oxide sintered body. The oxide sintered body has an InGaO3 phase which satisfies the relation [InGaO3]≥0.05.