PURPOSE: To improve a yield by raising a gas that is separated from a gas/liquid mixed fluid in a gas/liquid contact tower, bringing it into contact with a liquid that drops from a tower top condenser, discharging it, guiding a liquefied condensation constituent and a separated liquid to a storage drum, and the same time partially reducing the pressure for use as the refrigerant of the tower top condenser.

CONSTITUTION: A feed gas 1 with carbon dioxide gas as a main constituent is compressed by a compressor 1, passes through a cooler 2, a dewatering device 3, and a tiller 4 and becomes a gas/liquid mixed fluid, and is subjected to gas/ liquid separation by a surge tank 5. A gas flows into the lower part of a gas/ liquid contact tower 7 and rises after passing through packings 71 that are packed, is condensed by a tower top condenser 72 and touches a liquid that flows down, and separates a liquid condensed constituent and is delivered from the top of a tower. Meanwhile, he liquefied, condensed constituent and the separated liquid are guided to a storage drum 6 and at the same time the pressure is partially reduced before being sent to the tower top condenser 72, thus increasing the amount of product by 5-10% for a constant feed gas and saving equipment and operation costs.