[Problem] To enable efficiently grinding a workpiece of a hard brittle material, a difficult-to-cut material, or others at a moderate high load.;[Solution Means] When surface-grinding a workpiece by a grinding wheel, a grinding load is monitored while the grinding wheel is reduced in cut-in speed when the grinding load rises and the cut-in speed is increased when the grinding load falls. Respective cut-in speeds with which the grinding wheel has a slower cut-in speed at a larger grinding load have been set in a manner corresponding to a plurality of respective load thresholds of the grinding load, and after starting grinding at a predetermined speed, the grinding wheel is decelerated or accelerated to a corresponding cut-in speed every time the grinding load rises or falls to a predetermined load threshold.
