This paper describes the tool wear mechanism involved when a cBN tool cuts hardened steel (SUJ2, HRC60). The wear mechanism differs when the cutting speed is above or below 75 m/min. When the cutting speed is below 75 m/min, mechanical wear seems to be the major cause of tool wear. Conversely, above 75 m/min (i.e., in the normal cutting range for cBN tools), substances adhere to the tool wear surface. As the speed increases above this 75 m/min, the adhering substances and wear also increase. During research, when such adhering substances were analyzed, not only were components of the workpiece (Fe) and tool detected but oxygen was found to be distributed similarly to Fe; and FeO was detected in the surface layer of the adhering substances. When a diffusion test was conducted on FeO and a tool, the FeO reacted with the tool's binder. Thus, when the cutting speed is above 75 m/min, the Fe in workpieces apparently reacts with tool binder and oxygen in the air, forming an adhering substance. As a result, adhesive wear is generated.