Diamond tools wear easily under cutting tungsten carbide. To clarify the wear mechanism, the authors composed a temperature-measurement system of a cutting point using a dual-colorinfrared pyrometer and performed planing experiments. Infrared rays, emitted from the contact point between a mono-crystal-diamond tool and a cobalt-free tungsten carbide, are transmitted though the diamond tool and an optical fiber and then they are detected by the pyrometer. Before the planing experiments, rubbing experiments were performed using a mono-crystal-diamond stick and a tungsten-carbide disk. The effects of gas environments and rubbing conditions on contact-point temperature, friction coefficient, and diamond wear were experimentally investigated. Planing experiments of the tungsten carbide using mono-crystal-diamond tool, were performed. The effects of planing conditions and gas environments on cutting-point temperature and tool wear were investigated. Through the experiments the following results were obtained. Rubbing and cutting point temperature is the highest in Argon gas followed by Nitrogen gas and is the lowest in Air. Diamond-tool wear is the greatest in Argon gas, less in Nitrogen gas, and the least in Air. The reason for this is that a chemically or physically absorbed layer of oxygen or nitrogen on tungsten carbide acts as a lubricant at the contact point. Cutting-point temperature was in proportion to cutting speed. The temperature under cutting speed at 90m/min and cutting depth at 1.0μm in Air was approximately 170degrees Celsius.