This paper clarifies the mechanism of determining the tool electrode wear ratio in electrical discharge machining (EDM) using spectroscopic measurement of the vapor density of the tool electrode material. The relative density of copper vapor in EDM arc plasma was obtained from radiant fluxes emitted from the arc plasma and the temperature of the arc plasma which was obtained from the radiant fluxes of two different wavelengths using the line pair method. It is known that a longer pulse duration results in a lower tool electrode wear ratio and a thicker carbon layer precipitated on the tool electrode surface. On the other hand, the density of copper vapor evaporated from the tool electrode surface was found to be lower when the carbon layer was thicker. Thus it was concluded that wear of the tool electrode is prevented owing to the protective effect of the carbon layer.