PURPOSE: To improve roughness of the polishing plane of a silicon wafer which influences device characteristics and to enable polishing down to 0.08nm or less with an average roughness of Ra by regulating the charge of abrasive grains in a working liquid within a specified range.

CONSTITUTION: Before an Si wafer 4 is mirror-worked, the charge of abrasive grains 7 in a working liquid 3 is regulated at 0.1mV to 30mV. For example, a liquid with a charge of abrasive grains 7 of 0.1-30mV where abrasive grains 7 such as of colloidal silica with an average diameter of 20-40nm are dispersed in a KOH aqueous solution of pH9-11 is used as the working liquid 3. The charge of abrasive grains 7 can be measured easily by a potential measuring instrument by electrophoresis. Si wafers 4 diced thin out of a single crystal Si ingot are mirror-worked by the following: supplying the working liquid 3 between them and a polishing cloth 2 fixed on a surface plate 1; applying a load 5 on the Si wafers 4 to give a relative speed 6 to the surface plate 1 and the Si wafers 4; and contacts of the Si wafer 4 with abrasive grains 7 working liquid 3.