PURPOSE: To effect a specified aeration performance from the initial stage of operating apparatus by supplying an aeration gas from the gas phase side of a gas permeable membrane and filling at least the gas phase side with the gas before liquid to be treated is passed on the liquid phase side of the membrane.

CONSTITUTION: An aeration gas is supplied continuously on the gas phase side 10b of a gas permeable membrane 11 before liquid to be treated is passed on the liquid phase side 10a of the membrane 11. With the gas phase side 10b at least filled with the gas, a valve mechanism 16 is shifted to a closed state, and then liquid to be treated is introduced on the liquid phase side of a membrane module 10 from an introduction line 12. From the difference in the partial pressure ratios of the gas between the liquid phase 10a of the module 10 and the gas phase 10b, a gas to be removed in the liquid to the treated moves from the liquid phase side 10a to the gas phase side 10b through the gas permeable membrane 11, while the aeration gas moves to the liquid phase side 10a, so that the gas to be removed is eliminated from the liquid to be treated. In this way, a gas other than the aeration gas is prevented from transferring to the liquid to be treated through the gas permeable membrane in the initial stage of the aeration treatment.