PURPOSE: To enable maintenance of a closed space at a desired air pressure and maintenance of the concentration of carbon dioxide in the space at an allowable level by venting only the necessary minimum air of which the concentration of carbon dioxide is relatively high while recirculating the air of which the concentration of carbon dioxide is low.

CONSTITUTION: A sensor 40 generates a signal showing the relative concentration of carbon dioxide in a cabin 11 and this signal is sent to a carbon dioxide meter 51 through a fuselage 50. An analog signal generated by the carbon dioxide meter 51 is inputted to a valve controller 53 via a fuselage 52'. The valve controller 53 generates an air control signal (depending on a valve 29) and this is carried to the three-way valve 29 through a line 54 to control the quantity of air to be vented overboard. In response to the air pressure in the cabin 11 measured by the air pressure sensor, an air pressure controller 57 makes an air pack 37 supply pressurized fresh air in larger or smaller quantity from outside the cabin. In the case when the air pressure in the cabin drops below the set minimum limit, the air pressure controller 57 generates a top priority signal to maintain the air pressure in the cabin at a desired level.