Patent ID: 8492300

Claim:
An insert comprising: a silicon nitride sintered body, including β-Si 3 N 4 as a main component; Mg; and a rare-earth element Re (Y, La, Ce, Er, Dy, Yb), a content of Mg in terms of MgO being 1.0-7.0 mol %, a content of Re in terms of an oxide thereof being 0.4-1.0 mol %, and a total content of Mg and Re being from 1.7 to less than 7.5 mol %, wherein the insert has a graded composition in which oxygen content increases from a surface of the sintered body toward an inside thereof such that 0.8-1.5 mass % of oxygen is contained in a region of less than 0.5 mm inside from the surface, 1.1-2.3 mass % of oxygen is contained in a region of 0.5 mm or more inside from the surface, and a difference in oxygen content between the regions is 0.1-1.0 mass %.