PURPOSE: To better and improve a grinding property and a grinding accuracy, by performing the flat grinding of the body to be ground inside a carrier ring while dressing the flat grinding faces of at least two grinding grindstones with the rotation of a dresser.

CONSTITUTION: A ring like dresser 19 is mounted on the circumferential track passing the about center part of the flat grinding faces 3a, 3a of at least two grinding grindstones 3, 3 over thereon and also the body W to be ground of the carrier ring 11 inside fixed respectively on the flat grinding faces 3a, 3a while dressing the flat grinding faces 3a, 3a by rotating the dresser 19 together with the grinding grindstones 3, 3. With this structure the flatnesses of the flat grinding faces 3a, 3a are maintained at high accuracy, the grinding properties of the bodies W to be worked in various shapes of comparatively small sizes can be bettered and improved and the grinding accuracy can also be improved.