PURPOSE: To certainly remove only foreign matter contained in a powder composed of various particle sizes while preventing the removal of the regular powder and to reconcile the lowering of the inferior product generating ratio of a final product and the enhancement of the production yield of a raw material powder.

CONSTITUTION: A high speed tool steel powder containing slag and showing a normal distribution mode in its particle size is divided into ten groups using sieves with mesh sizes of 32, 32/48, 48/60, 60/80, 80/100, 100/145, 145/200, 200/250, 250/350 and 350 and each of the groups is applied to a cyclone type air classifier and the lightweight slag powder taken out of the upper part of the classifier is discarded and only the heavy high speed tool steel powder taken out of the lower part of the classifier is recovered. The powders recovered from the respective groups are mixed to prepare a raw material which is, in turn, subjected to injection molding. No inferior product is generated in a product at all and the total amount of the powder used as the raw material is almost the same as the total amount before the removal of slag.