PURPOSE: To realize the uniform wear of a grinding material layer without reducing the biting-in force of the grinding material layer by forming the outer periphery surface portion out of the grinding material layer of a comparatively hard grinding material layer, forming the inner periphery surface portion out of a comparatively soft grinding material layer.

CONSTITUTION: A hard layer 2 is formed at the outer periphery portion of a grinding material layer 1, and a soft layer 3 is formed at the inner periphery portion of the grinding material layer 1. Grooves 4 for distributing cooling water that is to be discharged through the center opening 5 of the grinding material layer 1 onto the whole surface of the grinding material layer 1 are formed. The outer periphery surface portion consisting of the hard layer 2 is made so as to occupy the approximate 20-40% of the radius of the grinding material layer 1, and the portion of the inner periphery surface is made so as to occupy the rest of the whole surface area of the grinding material layer 1. In the case of being smaller than 20%, the effect of forming the hard layer 2 at the outer periphery is small, and in the case of being larger than 40%, the ratio of the hard layer 2 is too high, and sharpness is reduced. As a result, at the rotary speed of 2000-4000rpm, the wear degree of the outer periphery surface portion of the hard layer 2 can be made approximately the same as the wear degree of the inner periphery surface portion.