During pumping down of water molecules in a typical laboratory vaccum chamber (evacuation time constant: τ_p<1 s), we obserbed that the pressure decays exponentially on a time scale of sub-kilo seconds before it starts to decrease with nearly t⁻¹ time dependence. On the basis of the Henry-type adsorption-desorption rate equation on the chamber wall and the equation of evacuation, we discuss here that such an exponential decay curve can be realized in the quasi-steady state for pressure by the mathematical phase-space analysis. By applying this analysis to obserbed decay curves, we also show that the pressure decay is dominated by the chemisorbed water with an activation energy for desorption of ∼890 meV, which is in good agreement with the recent results of water adsorbed on oxide surfaces as investigated by surface science techniques. Our results establish a firm basis to describe evacuation process.