A translucent polycrystalline sintered body is mainly composed of an alumina, is suitable for the manufacture of an arc tube for a high-intensity discharge lamp, and has an average grain diameter of 35 to 70 μm, preferably 50 to 60 μm. In a case where the sintered body is in a 0.5-mm-thick flat plate shape, the in-line transmittance of the sintered body is 30% or more, preferably 50% or more. In this case, the ratio between maximum and minimum in-line transmittance values is 2:1 to 1:1 in the visible region of 360 to 830 nm. The bending strength of the sintered body is 250 MPa or more, preferably 300 MPa or more.