their respective intermediate layers and surface layers had the same composition and in which the sum t 1 t 2 of the thicknesses of the intermediate layer and the surface layer in examples 5 and 12 were 0 3 mm and 0 48 mm respectively no large difference in maximum load without crack was seen either between examples 8 and 13 in which their respective intermediate layers and surface layers had the same compositions and in which the sum t 1 t 2 of the thicknesses of the intermediate layer and the surface layer in examples 8 and 13 were 0 3 mm and 0 45 mm respectively in each of those examples in which the value of t 2 t 1 t 2 was not less than 0 2 but not more than 0 5 the maximum load without crack thereof exceeds 110 mpa in making a comparison between example 7 and comparative example 18 in which their respective intermediate layers and surface layers had the same composition the value of t 2 t 1 t 2 in example 7 and in comparison example 18 were 0 27 and 0 56 respectively and the maximum load without crack in example 7 and in comparison example 18 were 130 mpa and 100 mpa respectively thus a large difference in maximum load without crack was seen between example 7 and comparative example 18 and therefore it has been confirmed that the value of t 2 t 1 t 2 exerts a great influence on the fatigue strength.