To solve the problem associated with an alumina sintered compact conjugated with other components, wherein occurrence of light scattering at boundaries between the alumina sintered compact and the other components makes it difficult to obtain a highly translucent sintered compact.

In a composite sintered compact comprising alumina and an aluminate compound which has a -alumina structure and has a refractive index equivalent to that of alumina, the amount of the aluminate compound having the -alumina structure is adjusted to 1-50 wt.% so as to obtain a highly translucent, high-strength alumina sintered compact which shows 45% total light transmission at the wavelength of 600 nm and 500 MPa flexural strength for a 1 mm-thick sample. The sintered grain size is preferably 10 m.