Rail surface defects called shellings in Japan or squats in Britain, hereinafter referred to as surface shellings, are a kind of rolling contact fatigue(RCF) defects causing considerable maintenance cost including rail renewal not only on Shinkansen lines but also on narrow gauge lines. Preventive grinding of rail head surface is currently most reliable and effective as a measure to reduce the occurrence of surface shellings so that a lot of railway companies adopt it. However, since the effect of the preventive grinding is not made clear enough, it is of great interest to obtain the optimal grinding period and grinding amount (thickness from rail surface). In this paper the effect of the preventive grinding for Shinkansen rails is discussed on the basis of experimental results using a large rolling disc machine.