PROBLEM TO BE SOLVED: To provide a gas treatment device capable of realizing energy-saving control and humidity decrease control while emphasizing gas treatment capacity.SOLUTION: A gas treatment device has a mixing ratio applied voltage changing unit 9. In the mixing ratio applied voltage changing unit 9； a mixing ratio M controlled by a mixing controlling unit 7 is changed within a range from a practical upper limit mixing ratio Mmax to a practical lower limit mixing ratio Mmin on condition that current treatment capacity exhibited by a plasma treatment section 3 is maintained in a situation that a difference Δh (Δh=hF-hR) between first absolute humidity (absolute humidity of a mixing treating object gas D) hF and second absolute humidity (absolute humidity of a treated gas E) hR exists in a reference humidity difference range ΔhB±α； and a voltage V applied between electrodes in a plasma treatment section 3 is changed within a range from a practical upper limit voltage Vmax to a practical lower limit voltage Vmin.SELECTED DRAWING: Figure 1
