PURPOSE: To improve density, strength and hardness of composite sintered compact by sintering a powdery composition containing hexagonal boron nitride powder, boron silicate powder, silicon carbide powder and sintering assistant.

CONSTITUTION: A powdery composition containing 20-70 pts.wt. hexagonal boron nitride powder, 20-70 pts.wt. silicon nitride powder, 4-35 pts.wt. silicon carbide powder as 100 pts.wt. total amount and simultaneously further containing 1-10wt.% sintering assistant based on the above-mentioned silicon nitride is sintered. As the above-mentioned sintering assistant, rare earth oxide or mixture containing the rare earth oxide is preferably used and mixture of ytteria (Y2O3) powder and alumina (Al2O3) are preferably used, since the mixture is especially excellent from the viewpoint of sintering properties, stability and strength.