PURPOSE: To perform cooling of a granular material simultaneously with polishing and to improve working efficiency by a method wherein the open air sucked through the suction force of a dust collecting device is brought into direct contact with water to produce cooled air, which is blown in a processing tower for polishing.

CONSTITUTION: When the temperature of particles of sand intended to be processed is high and an atmospheric temperature is also 30°C or more, air for cooling is fed to a processing tower 1 from a cooling unit 2. Namely, air delivered from the outside by means of a cooling chamber 12 is brought into direct contact with water for cooling, and the air deterred by a feed chamber 13 is fed to the processing tower 1 through a feed pipe 13g. The feed of cooling water is effected through utilization of scanning of the interior of the processing tower 1 by means of a dust collecting device 4. As noted above, by feeding cooling air to the processing tower 1, the particles of sand can be cooled simulta neously with polishing even when processing may be effected in a high tempera ture state as in casting sand.