An air purifying system is a system for purifying air in a room, and the air purifying system includes: a carbon dioxide removing device including a first space and a second space that are divided from each other by a separation membrane that selectively allows carbon dioxide to permeate therethrough; a feed passage that leads the air in the room to the first space; a return passage that leads purified air from which carbon dioxide has been removed from the first space to the room; a supply passage that supplies sweep gas to the second space, the sweep gas having a carbon dioxide partial pressure that is lower than a carbon dioxide partial pressure in the air in the room; and a discharge passage that discharges the sweep gas from the second space after the sweep gas is mixed with the carbon dioxide that has permeated through the separation membrane.