PURPOSE: To obtain the sintered body which has high density and is high in rectilinear transmittance even in a large-sized material and uniform by making yttria powder whose purity and specific surface area are regulated to the specified value dense with a hot press in the vacuum at the specified conditions and thereafter performing HIP treatment therefor at the specified conditions.

CONSTITUTION: Yttria powder having ≥99.9% purity and ≥10m2/g specific surface area (BET value) is made dense at ≥95% theoretical density ratio by a hot press in the vacuum at 1300-1700°C at 100-500kg/cm2. Then the aimed yttria sintered body is obtained by performing HIP treatment therefor at 1400-1900°C at ≥500kg/cm2. A sintering adjuvant is unutilized and therefore this sintered body is optically made uniform and also high density and has extremely excellent rectilinear transmittance in a visible region and an infrared region. Furthermore this sintered body is optically uniform even in a large-sized material and preferable for an infrared transmission window, etc., utilized at thickness not less than 3mm.