An air system in which the supply air from a compressed air source is supplied to a pneumatically driven device through a pneumatic line, including humidity conditioning tubes used for the whole or a part of the pneumatic line. This humidity conditioning tube includes a polymer material having moisture selective permeability in both inward and outward directions according to steam partial pressure in and outside thereof, and the supply air supplied to the pneumatically driven device through the tube is dehumidified or humidified by this humidity conditioning tube. An air system can suppress generation of mist or the like in supply air without shortening a life of a device caused by supply of low dew-point air to the pneumatically driven device.