There is provided a method for producing an a-IGZO oxide thin film by sputtering, which can control the carrier density of the film to a given value with high reproducibility. The method is an amorphous In—Ga—Zn—O based oxide thin film production method including: providing a sintered oxide material consisting essentially of indium (In), gallium (Ga), zinc (Zn), and oxygen (O) as constituent elements, wherein the ratio [In]/([In]+[Ga]) of the number of indium atoms to the total number of indium and gallium atoms is from 20% to 80%, the ratio [Zn]/([In]+[Ga]+[Zn]) of the number of zinc atoms to the total number of indium, gallium and zinc atoms is from 10% to 50%, and the sintered oxide material has a specific resistance of 1.0×10−1 Ωcm or less; and producing a film on a substrate by direct current sputtering at a sputtering power density of 2.5 to 5.5 W/cm2 using the sintered oxide material as a sputtering target.