Ar gas effects on the field-emission (FE) properties of multiwalled carbon nanotubes (CNTs) were investigated. The FE measurement was carried out in vacuum and in Ar gas using a scanning-electron microscope (SEM) chamber with a removable gas cell fitted in the SEM chamber. The gap between electrodes was set by observing a SEM image. The electrodes were characterized in situ before and after FE measurements by SEM observation. The FE properties of CNTs in Ar gas at 80 kPa and lower pressures were the same as those in vacuum before and after the measurements in gas. At pressures more than 80 kPa, the FE threshold voltages in Ar gas were higher than those in vacuum and increased with increasing Ar gas pressure. However, the FE properties in vacuum after the measurement in gas were the same as before. When the FE threshold voltages became higher than the voltage for gas breakdown, a gas discharge was ignited before the FE appearance. After the gas discharge, the carbon nanotube cathode was damaged.