sprag 3 was formed on the surface of the sprag 3 in order to aggregate carbon elements on the surface of the sprag 3 next in an embodiment 2 the surface of the sprag 3 was coated by diamond like carbon dlc in order to aggregate carbon elements on the surface of the sprag 3 further in an embodiment 3 electroless nickel plating was applied to the surface of the sprag 3 rather than the carbon coating further as an embodiment 4 similar to the embodiment 2 in order to aggregate carbon elements on the surface of the sprag 3 the surface of the sprag 3 was coated by carbonized tungsten carbon wc c it was found that the result of the stroking test for the embodiment 4 was substantially the same as the result in the embodiment 2 shown in fig2 in a comparative example the suj2 material and the m50 material were used and no coating was applied to the surface of the sprag 3 here with reference to fig2 the results of the stroking tests performed under the following conditions in the comparative example and the various embodiments of the present invention will be explained incidentally a bar graph represents the number of cycles on the limit at which the engaging ability is considerably reduced in the comparative example wherein the sprag which was not subjected to the surface treatment regarding both the suj2 material and the m50 material the engaging ability was considerably reduced at the number of cycles smaller than about 300000 to the contrary in the embodiment 1 the engaging ability was maintained till about 2000000 cycles further in the embodiment 2 the engaging ability was maintained till more than about 4000000 cycles in the embodiment 3 the engaging ability was maintained.