A silicon nitride sintered body comprising Mg and at least one rare earth element selected from the group consisting of La, Y, Gd and Yb, the total oxide-converted content of the above elements being 0.6-7 weight %, with Mg converted to MgO and rare earth elements converted to rare earth oxides RE_xO_y. The silicon nitride sintered body is produced by mixing 1-50 parts by weight of a first silicon nitride powder having a -particle ratio of 30-100%, an oxygen content of 0.5 weight % or less, an average particle size of 0.2-10 m, and an aspect ratio of 10 or less, with 99-50 parts by weight of -silicon nitride powder having an average particle size of 0.2-4 m; and sintering the resultant mixture at a temperature of 1,800 C. or higher and pressure of 5 atm or more in a nitrogen atmosphere.