Supplying, wherein the carbon dioxide supply device 10 is driven for two minutes (a first-period specified in advance) during which the ventilation device 31 is stopped, and spreading, wherein, after the supplying being executed, the carbon dioxide supply device 10 and the ventilation fan 31 are stopped for two minutes (a second-period specified in advance), are alternately repeated. When the temperature T in the agricultural greenhouse AH reaches T2 or more during one of the supplying and the spreading is being executed, the subsequent driving of the ventilation fan 31 is inhibited until end of execution of the spreading.
