A sintered compact essentially consisting of zinc (Zn), gallium (Ga), silicon (Si) and oxygen (O), wherein a Zn content expressed in terms of ZnO is 5 to 60 mol %, a Ga content expressed in terms of Ga2O3 is 8.5 to 90 mol %, and a Si content expressed in terms of SiO2 is 0 to 45 mol %, and the sintered compact satisfies a condition of A≤(B+2C) when the Zn content expressed in terms of ZnO is A (mol %), the Ga content expressed in terms of Ga2O3 is B (mol %), and the Si content expressed in terms of SiO2 is C (mol %), and has a relative density of 90% or higher. An object of this invention is to efficiently obtain an amorphous film having high transmissivity and a low refractive index without having to introduce oxygen into the atmosphere during film deposition by DC sputtering.