PURPOSE: To obtain the titled powdery raw material of high purity having a small particle diameter, by powdering a mixture solution of silica sol with alumina sol and a slurry carbon by hot-air dyring, etc., and subjecting the resultant powder to the synthetic reaction in a nonoxidizing gas atmosphere under heating.

CONSTITUTION: A mixture solution is prepared from silica sol, alumina sol and slurry carbon, preferably a carbon black slurry, to give the desired composition of silicon aluminum oxynitride type powder. In this case, the raw materials must be adjusted to give about ≤4.6pH difference between the respective starting raw materials. The resultant mixture solution is then subjected to the hot- air drying, e.g. spray drying, or gelling, hardening and pulverizing treatment, etc. to give the aimed powder. The powder is then subjected to a synthetic reaction at about 1,200W1,800°C in a nonoxidizing atmosphere containing N2 to afford the aimed titled powdery raw material, i.e. sialon type powdery raw material.