Recently, KSTAR, Korea's superconducting fusion energy research and development device, has succeeded in driving the high performance plasma for 70 seconds for the first time in the world. Continuous plasma operation technology is an essential factor for commercialization of fusion energy power generation. Therefore, this achievement is expected to play a major role in the research of fusion technology required for future fusion power plants. In order to operate the KSTAR, the discharge process in which the neutral gas is turned into the plasma should be preceded in the start-up (breakdown) phase of tokamak operation. This process essentially involves the vacuum environment in the tokamak device. KSTAR has successfully operated a vacuum pumping system to achieve the target level of the vacuum environment through a high temperature baking operation, a discharge cleaning process and boronization.