By using Müller-type field emission microscope with an ordinary pumping system, the effects of gas adsorption and field desorption on the carbon-adsorbed tungsten surface were examined at high temperature around 1500°K. By introducing a very small quantity of oxygen into the field emission microscope, it was found that the clean-up of the carbon-adsorbed surface of the tungsten-tip was observed. Carbon adsorption was caused by using the tip under conventional operating condition in high vacuum. This carbon-adsorption process is discussed with pictures taken successively during the process. From the results of such observations, it is shwon that the change in the 011 and 111 region is always regular, the 001 region showing the very complicated change. The field desorption of the carbon-adsorbed tungsten surface was also observed, and it was confirmed that the same change as obtained in the gas introduction was seen in the 001 region.