PURPOSE: To provide good grinding surface and stabilize the amount of grinding per unit time by keeping the gap between a grinding tool and a work to be ground at the target value, wherein control for keeping the gap is conducted on the basis of dynamic pressure generated between the grinding tool and the work.

CONSTITUTION: The gap t formed between a grinding tool such as polyurethane sphere 6 and a work to be ground 18 relates to the dynamic pressure P generated in this gap as 1:1. This requires determination of the dynamic pressure P' corresponding to the target gap t' from the relation between the dynamic pressure P and the gap t determined on a preliminary experiment. The load on the grinding tool 6 is controlled on the basis of deviation of the sensed dynamic pressure p from the dynamic pressure P'. This maintains the gap t at all times at the target value t' to prevent contact of the grinding tool 6 with the work 18, that provides good ground surface deterioration at layer due to processing.