completely saturated on the contrary as can be seen in the results of test shown in fig1 when the carbon concentration of the retainer falls below 0 9 by weight the percent retainer deformation suddenly rises rendering the retainer more subject to deformation by centrifugal force it is thus made obvious that when the upper limit and lower limit of the surface carbon concentration of the retainer are predetermined to be 1 2 by weight and 0 9 by weight respectively the retainer can be prevented from deforming by its own weight or by the weight of the rollers during high speed rotation subsequently the inventors prepared a retainer shown in fig9 from the same steel material as that of sample nos 1 to 5 and 9 specimens as shown in table 3 the retainer was incorporated in the loading cam device which was then subjected to retainer deformation test at a rotary speed of 6 000 min u2212 1 for 6 hours the results of test are shown in fig1 in fig1 the abscissa indicates surface nitrogen concentration and the ordinate indicates abrasion ratio relative to the abrasion of sample no 9 specimen as 1 the inventors further made charpy impact test on sample nos 1 to 5 and 9 specimens as shown in table 3 according to jis z 2242 the.