PURPOSE: To enable always obtaining the same level of working precision as that attainable by direct automatic sizing even if a plurality of grinding wheels are present on a single wheel spindle stock moving axis, and to perform position control by making correction on the basis of the abrasion amount of dressing diamond tool so as to enable accurately obtaining the positional relationship between the fore end of the diamond tool and the outer periphery of the grinding wheel at the time of dressing.

CONSTITUTION: By measuring the position of a diamond tool tip by a measuring instrument for each dressing operation, the abrasion amount of the diamond tool tip is determined and this abrasion amount is fed to a dressing co-ordinate offset value calculation section 43 and to a grinding wheel data calculation section 44 to thereby make corrections in these two sections, whereby the zero offset value is corrected using the grinding wheel data when grinding is performed to thereby make the position control. Further, when the dressing is conducted, the zero offset value is corrected using the grinding wheel data and the dressing co-ordinated offset value, thereby effecting the position control based on the consideration of the influence upon the position of the grinding wheel abrasion amount and the diamond tool abrasion amount.