High efficiency of gas absorption may be one of the important techniques for protection of global environment. A new concept of gas absorption by utilizing condensation process was proposed which is called "condensate film method", and its proof experiment was conducted with regard to the condensation heat transfer in vertical cooling tube. Theoretical analysis was performed on the critical condition of mist formation in the cooling tube flow and the falling liquid film under the same mass flow rate condition. Comparing between the both analytical results and experimental data, the concept proposed in this paper was identified and the mechanism of the gas absorption through the condensate and the mist droplets was clarified.