PURPOSE: To reduce the dispersion of the working cycle time due to the change in the cutting performance of a grinding wheel, as for a grinding device.

CONSTITUTION: A worked state calculating means 140 calculates a plurality of factors showing the worked state which changes according to the change in the cutting performance of a grinding wheel 19, and a forestage grinding completion diameter correcting means 150 performs correction so that the previously determined forestage grinding completion diameter is set close to the target finished diameter according to the degree of the excellent cutting performance of a grinding wheel through the fuzzy estimation, using a plurality of factors as input information. A control means 130 grinds a ground surface by the grinding wheel by advancing the wheel spindle stock 13 up to the time when the outside diameter of the ground surface Wa becomes equal to the forestage grinding completion diameter corrected by the forestage grinding completion diameter correcting means, and then the wheel spindle stock is advanced at a fine grinding speed, and the ground surface is ground by the grinding wheel.