[en] The origin of neutral gas species outside a plasma in the JFT-2 tokamak was investigated and its correlation to light impurities in a plasma was studied. The principal gases measured by a quadrupole mass spectrometer during and after a discharge are water, carbon monoxide, and methane, whose partial pressure rises are of the order of 10^-7 torr. The wall supplies oxygen impurity to the plasma in the form of water vapour. The limiter mainly contributes to the production of carbon monoxide and methane. There is no relationship of light impurities to methane and carbon monoxide. As for carbon and silicon carbide limiters, the formation of methane after a discharge results from the chemical reaction of implanted hydrogen atoms with the hot limiter. (Auth.)