PURPOSE: To make it possible to facilitate the detection of the starting time of a grinding action by a grinder by providing a light emitting element radiating light rays outside a visible region and an optical detector through a gap between the grindstone and a workpiece and by controlling the access motion of the grindstone to the workpiece by the amount of received light.

CONSTITUTION: Grinding work is initiated by a method wherein a grindstone 1 is made to some close to a workpiece 3, the access motion is once stopped when the length L of a gap between said grindstone and workpiece reached a minute set value, a grinding liquid, etc., is set to a working state, and grindstone feed is set in grindong feed mode. A light emitting element 5 radiating light rays outside a visible region and an optical detector 6 are provided through said gap, and displays on an actinometer 8 and an electric light indicating part 9 are changed by the amount of light of the optical detector 6. When the gap length L approaches a set value, a display on the electric light indicating part 9 changes from blue to yellow, and to red at the set value. Hereby, the access motion of a grindstone can be controlled.