An oxide sintered body which is obtained by mixing and sintering zinc oxide, indium oxide, gallium oxide and tin oxide. The relative density of the oxide sintered body is 85% or more and the average grain size of crystal grains observed on the surface of the oxide sintered body is less than 10 μm. X-ray diffraction of the oxide sintered body shows that a Zn2SnO4 phase and an InGaZnO4 phase are the main phases and that an InGaZn2O5 phase is contained in an amount of 3 volume % or less.