Ultra-low concentrations of impurities such as water in a highly-purified gas are analyzed by a system having an ion source chamber and a drift chamber. The ion source chamber ionizes one of a sample gas and a carrier gas to produce main component ions, and the other of the sample gas and carrier gas is introduced into the drift chamber. The invention controls the residence time of main component ions in one of the first and second chambers to be shorter than the mean reaction time of main component ions and impurity molecules of the sample gas in the one of the first and second chambers.