PURPOSE: To obtain CO by removing CO 2 , by removing N 2 from a gaseous mixture containing at least N 2 , CO and CO 2 by a pressure swing type separation method, and succeedingly applying a solution absorbing type decarbonizing separation method.
CONSTITUTION: Stock gas (a) is supplied to an adsorbing tower A by bringing a vacuum state to a pressure rising process by opening a valve 1. Valves 3, 6 are opened and hardly adsorbable gas is recovered in a buffer tank D. The valve 7 provided to the connection pipe to an adsorbing tower B to reduce the internal pressure of the adsorbing tower A to atmospheric pressure. A valve 9 is opened to enter a purge process of the hardly adsorbable component in the gap of the adsorbing tower A. A valve 11 is opened while a valve 7 is closed and vacuum evacuation is performed by using a vacuum pump to recover easily adsorbable CO 2 and CO being product gas. In absorbing tower F, a part of carbon dioxide and carbon monoxide are physically absorbed with a polyethylene dimethyl ether solution to obtain high purity carbon monoxide.