To provide a gas analyzing method and its device which is compact and easy to maintain.

The measurement device comprises a diffusion scrubber 1, a temporary liquid reservoir 2 which supplies the diffusion scrubber 1 with pure water as adsorbent liquid, a tank 3 where the pure water over-flown from the temporary liquid reservoir 2 is held, a pure water device 4 which regenerates the pure water in the tank 3, and ion chromatograph 5 which performs component analysis of the pure water running through the diffusion scrubber 1. Relating to the pure water device 4, a positive ion exchange film 8 and a negative ion exchange film 9 are assigned between an anode 6 and a cathode 7, and between the cathode 7 and the positive ion exchange film 8 and between the negative ion exchange film 9 and the anode 6, dilute nitric acid is introduced from the ion chromatograph 5. By performing ion exchange between NH4+ ion in the pure water and H+ ion in the dilute nitric acid with the assistance of electric permeability due to positive ion exchange film 8 and electric poles 6 and 7, pure water is regenerated and fed to the temporary liquid reservoir 2.