PURPOSE: To cool a grinding wheel surely by furnishing No.1 nozzle, which is to inject the cleaning liquid in the direction along with rotation of the grinding wheel, and No.2 nozzle which is located in the neighborhood thereof and injects a high pressure coolant in the direction opposite to the rotation.

CONSTITUTION: A work W is placed on a working table 10 and ground by a large grinding wheel 12, wherein the jet 14 of No.1 nozzle 13 is located facing that part of work W to be ground, and the jet 16 of No.2 nozzle 15 facing the periphery of the grinding wheel 12. Under grinding, high-pressure coolant is sprayed to the grinding wheel 12 from this jet 16 while low-pressure coolant is injected out of the jet 14 toward that part of work W to be ground. Accordingly no air layer A will be formed at the periphery of the grinding wheel 12 in its zone from the point whereto the high-pressure coolant is sprayed to the part ground, to ensure that the low-pressure coolant is certainly fed to the part to be ground. Also spraying of high-pressure coolant will detach minute grinding splinters from the grinding wheel 12. It is also acceptable to furnish No.3 nozzle 20. Thus the life of a grinding wheel can be prolonged.