PURPOSE: To perform a proper dressing and to perform a productive and highly efficient cutting, by deciding as the fall of an abrasive grain in the case of not reaching to the maximum load within the specific time and not reaching the work diameter value at the maximum load time at the work diameter in the specific time and dressing a grindstone after finishing.

CONSTITUTION: During the rough/coarse grinding of a trial grinding prior to a lot grinding the maximum load Wmax of a grindstone driving motor, the time T until reaching this load and the work diameter Do at this loading time are found and stored in a memory 17. In case of the work diameter Di at the time T not reaching the work diameter value Do, though reaching the maximum load Wmax within the time T during the grinding, it is decided as a grindstone clogging. In case of the work diameter Di at the time T not reaching the work diameter value Do without reaching the maximum load Wmax within the time T, it is decided as the fall of abrasive grains and the grindstone is subjected to dressing after the finishing.