PURPOSE: To set parallelism for a polishing surface and squareness to a work axis so accurately by rotating both upper and lower surface plates at equi- velocity in the same direction and thereby lapping a workpiece.

CONSTITUTION: A work 6 held by each carrier 3 is held between both upper and lower surface plates 1 and 2, while these plates are rotated at equi-velocity in the same direction as making these carriers 3 into planetary motion through a sun gear 4 and an internal gear 5 whereby both sides of the work 6 are lapped. At time, since these upper and lower surface plates 1, 2 are rotated at equi-velocity in the same direction, tensile force acting on both upper and lower surfaces of the work 6 comes to the same direction and the same size, so that any reverse tensile force will not act on both the upper and lower surfaces of the work 6, therefore such a possibility that the work 6 might be polished aslant is not longer caused to happen.