PURPOSE: To lessen the consumption of kinetic energy and increase grinding ratio by continuously installing polishing grinding wheels with high density and less pores, having numerous projections and recesses on the surface, and made of abrasive grains of carborundum, etc., in alignment on a shaft and forming a rotor type polishing body.

CONSTITUTION: Polishing grinding wheels 2 with high density and less pores, having many small projections and recesses, and made of abrasive grains of carborundum, alumina, etc. are continuously installed in alignment on a shaft 3, being formed into a rotor type polishing body 5, which is installed in a case 1. Grain objects are repeatedly brought into contact with the polishing faces of the polishing grinding wheels 2 in a narrow polishing zone 15 on the lower part due to the rotation of a prime mover 18. And, a force applied to the center direction of the grain object is less and the force is chiefly applied to the peripheral direction and, thereby, the grain objects are rotated, subjected to less impact, ground without being broken, and taken out in a low position. And, produced light powder bodies are separated in a wide space 21 and collected into a dust collector 8.