PURPOSE: To reduce the specific gravity to reduce the centrifugal force in the case of rotating a grindstone at high speed and to increase safety by using a porous silicon nitride ceramic for a grindstone main body.

CONSTITUTION: A grindstone main body is formed of silicon nitride ceramics whose apparent porosity is 15-25%, whose flexural strength is ≥10kgf/mm2 at a room temperature and whose thermal expansion coefficient is (2.5-3.5)×10-6/°C at a room temp.-1000°C. A multiporous type grinding layer consisting of the super hard abrasive grain of a cubic boron nitride or diamond, etc., and a vitrified binder is then provided on the outer peripheral surface of this grindstone main body and a desired super hard abrasive grain grindstone for a high speed grinding is obtained.