In the air conditioner, a mixed atmosphere in which carbon dioxide gas is mixed with the atmospheric gas in the mixing section (3) connected to the outlet (6) from the outlet (6) installed in the vicinity of the plant, and the temperature and humidity are adjusted. By supplying gas substantially uniformly to the plant, the transpiration of the plant is made uniform, and only the vicinity of the plant is air-conditioned and the carbon dioxide concentration is managed.空調装置において、植物の近傍に設置された吹出し口（６）より、吹出し口（６）と連通された混合部（３）で雰囲気ガスに炭酸ガスを混合しさらに温度及び湿度を調節した混合雰囲気ガスを植物に対し略均一に供給することで植物の葉の蒸散を均一にし、さらに植物の近傍のみを空調制御し、かつ炭酸ガス濃度を管理する。
