PURPOSE: To keep the parallelism of the upper and lower surface plates with high precision independently of the number of worked lots by always alternately changing the direction of rotation of a carrier in the normal and reverse directions by continuously varying the number of revolution of a sun gear and an internal gear in sinusoidal form, during work.

CONSTITUTION: A work holding carrier 12 is arranged between the upper and lower surface plates 11 and 6, and the upper and lower surface plates 11 and 6 are revolved at a revolution speed ratio of 3:1 in the opposite directions. Further, an internal gear 14 meshed with the carrier 12 and a sun gear 13 are revolved at the revolution speed which varies with the sinusoidal performance in the same direction, and the rotation direction is alternately changed in the clockwise and couterclockwise directions during lap work, revolving the carrier 12 in the same direction to the lower surface plate 6. Accordingly, a work 23 held on the carrier 12 is lapping-worked with the parallelism and flatness with high precision by the upper and lower surface plates 11 and 6.