PURPOSE: To improve fracture toughness of Si3N4 sintered body by incorporating the specified amount of long columnar grain having specified length in the crystal grain of β-Si3N4 sintered body which is obtained by using α-Si3N4 powder as a starting raw material.

CONSTITUTION: In β-silicon nitride (Si3N4) sintered body obtained by using α-silicon nitride (Si3N4) powder as a starting raw material, long columnar grain having ≥50μ length are incorporated at 5W50vol.% among β-Si3N4 crystal grain. The length of the long columnar β-Si3N4 crystal grain preferably is 50W100μ and the value of breaking toughness is lowered in case of ≤50μ length and on the other hand, bending strength tends to be lowered in case of ≥100μ length. Further when the long columnar β-Si3N4 crystal grain are less than 5vol.%, such probability that development of cleavage is allowed to collide against the long columnar grain is lowered and therefore enhancement of toughness is lowered and furthermore its toughness is not stabilized. On the other hand, in the case of ≥50vol.%, a cluster is formed by the long columnar grain and thereby bending strength is remarkably lowered.