PURPOSE: To enable efficient contact of gas and liquid by a simple means, by a method wherein, when liquid is permeated from s supply side of a permeation film device to a film permeation side, a gaseous substance is fed simultaneously and flowed through to the film permeation side.

CONSTITUTION: Liquid to be treated is fed under a certain pressure from a liquid feed pipe 1 to a permeation film device proper 2, and is permeated in a thin film state to the opposite side to that of a permeation film 3. Said liquid is gradually flowed as liquid drops 4 to a lower part. In the meantime, a gaseous substance is fed from a gas feed pipe 5, and rises as it contacts with liquid permeating through the permeation film 3. Said gaseous substance is then discharged through a treated gas discharge pipe 6. The liquid drops 4, which flow down as it contacts with the gaseous substance, dorp from the lower end of the permeation film 3 to be stowed in a treated liquid reserving tank 7 and discharged to the outside of a system through a treated liquid discharge pipe 8. A permeation film module constituted of elements, made of hollow filaments having a high packing density is used preferably.