Silicon nitride powder, silicon nitride sintered body, sintered silicon nitride substrate, and ci

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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 RExOy. The silicon nitride sintered body is produced by mixing 1–50 parts by weight of a first silicon nitride powder having a β-particle ratio of 30–100%, an oxygen content of 0.5 weight % or less, an average particle size of 0.2–10 μm, and an aspect ratio of 10 or less, with 99–50 parts by weight of α-silicon nitride powder having an average particle size of 0.2–4 μm; 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.