Provided is a diamond-coated sliding part that is light, has excellent abrasion resistance, that prevents abrasion of the material of an opposite member, and that is effective in reducing power loss. This sliding part is especially useful as an adjusting shim for the valve train mechanism of an internal combustion engine such as an automobile engine in which the base material is silicon nitride or sialon and this base material surface is coated with a diamond coating layer. By performing finish processing on only a small part of peaks of diamond particles protruding from the surface of the diamond coating layer to reduce the height of the protrusions, or by controlling film forming conditions, etc., the profile bearing length ratio (tp) at a cutting level of 0.1 &mgr;m for the sliding surface of the diamond coating layer is made 60% or greater.