To provide a method which gives a high speed of gas transportation between liquids, gasifies a liquid without resort to sparging bubbles through the liquid and, therefore, performs the gasification of the liquid efficiently without excessive stirring by the generation of air bubbles.

This method for allowing a gas to be added to or to be removed from a solution of the gas in the liquid is related to the transportation of the gas through a membrane unit between one liquid and another liquid. The membrane unit contains a membrane which is substancially impermeable to a solvent, has a permeability of oxygen for at least 1×107 Pa (100 bar), is formed with perfluoro-2,2-dimethyl-1,3-dioxole amorphous copolymer and is kept at a temperature which is less than the glass transition temperature of the copolymer. Therein, the fluid can be either another liquid or gas.