PURPOSE: To improve strength and toughness by blending starting raw materials for α'-Si3N4 and β-Si3N4 respectively represented by a specified formula with plural kinds of Si3N4 powders with mutually different particle diameter, thus preparing a raw material composition and subsequently sintering the resultant composition.

CONSTITUTION: Two more kinds of Si3N4 powders respectively having ≤1.2μm average particle diameter and satisfying that the maximum value of the deference between the average particle diameters ≥0.3μm are blended with starting raw materials for α'-Si3N4 represented by formula I (M is Li, Ca, Mg or Y; 0<x≤2) and β'-Si3N4 represented by formula II (0<z≤2), i.e., powdery AlN and or more powders selected from Li2O, CaO, MgO and Y2O3 in a prescribed ratio to prepare a raw material composition. The obtained raw material composition is then pressure-molded into a prescribed shape and subsequently sintered at about 1650-1900°C in an atmosphere of a non-oxidizing gas such as N2 gas, thus obtaining the objective Si3N4 sintered material composed of α'-Si3N4 and β'-Si3N4 and having a high strength and a high toughness.