Provided is an oxide sintered body that, when used to obtain an oxide semiconductor thin film by sputtering, can achieve a low carrier concentration and a high carrier mobility. Also provided is a sputtering target using the oxide sintered body. The oxide sintered body contains, as oxides, indium, gallium, and at least one positive divalent element selected from the group consisting of nickel, cobalt, calcium, strontium, and lead. The gallium content, in terms of the atomic ratio Ga/(In+Ga), is from 0.20 to 0.45, and the positive divalent element content, in terms of the atomic ratio M/(In+Ga+M), is from 0.0001 to 0.05. The amorphous oxide semiconductor thin film, which is formed using the oxide sintered body as a sputtering target, can achieve a carrier concentration of less than 3.0×1018 cm−3 and a carrier mobility of at least 10 cm2V−1 sec−1.