PURPOSE: To improve precision of corrected grinding by a method wherein when the surface plates of the lapping apparatus is worn, the upper surface plate out of the upper and lower surface plates is vertically rocked in the axial direction of a rotating shaft relative to the lower surface plate and rubbed together with the lower surface plate to correct the grinding.

CONSTITUTION: A grinding treatment for corecting irregular abrasions of the ground surfaces of the upper and lower surface plates 3, 4 is carried out by raising the lower surface plate 3 together with the upper surface plate to a position higher than that of a sun gear 5 by operating a cylinder member 34 associated with the lower surface plate 3, by deflating the compressed air in a hollow shaft 12, by rocking the lower surface plate 3 vertically in the axial directions of the rotating shafts of the surface plates 3, 4 by operating the cylinder member 34, and at the same time, by driving an electromotor 26. That is, when the lower surface plate 3 is raised, the upper surface plate 4 on the lower surface plate is placed at a high position where it can radially freely move and therefore, is rocked with the hollow shaft 12 and eccentrically rocked.