PURPOSE: To ventilate a windowless barn with a necessary minimum amount of air in winter and without consuming an unnecessarily large amount of electric power in summer, by controlling the amount of air to be used for ventilation of barn on the basis of temperature and amount of oxygen.

CONSTITUTION: Temperature detector 21 and oxygen detector 22 are provided in livestock breeding section 12 of barn 1, and on the basis of temperature and amount of oxygen detected by these detectors 21, 22, a required number of ventilating fans 8 are set into operation and ventilating rate is controlled by controller 20. When the fans 8 are set into operation, a required amount of air is introduced into livestock breeding section 12 from air inlet ports 4 formed in opposite walls 2 of barn 1. Then, air is carried from section 12 via a first exhaust port 5 to the back of ceiling 7 and then to the upper end of exhaust duct 10 via a second exhaust port 9. In the duct 10, foreign matters such as dust or the like contained the exhaust gas is removed by a net-like filtering means 19, and exhaust gas thus purified is discharged to the outside from a third exhaust port 11 located at the lower end of duct 10.