To effectively use an abrasive cloth and prolong a life of a dresser by dispensing with or restraining to be minimal an initial dress, and to perform precise working.

When the abrasive cloth 13 is replaced with a new cloth, a conversion factor for converting an integral value of a rotation torque of a surface plate 12 at working into a working amount is initially set by a trial working of a first workpiece, the integral value is found at working, and working of following workpieces are performed such that a target working amount found using the integral value and the conversion factor is achieved. The working amount by such a working is measured, and when the measured working amount is deviated from an allowable value of the target working amount, the conversion factor is corrected while taking a difference between both working amounts into consideration.