PROBLEM TO BE SOLVED: To suppress the amount of NO generated, by suppressing wasted application of nitrogen components, through application of nitrogen components according to the status of absorption of nitrogen components by plants.SOLUTION: The method includes the steps of: continuously monitoring the amount of NO generated from plant cultivation beds after application of nitrogen components with the monitoring result as an index, diagnosing the status of absorption of nitrogen components by plants in real time and controlling the amount of nitrogen components to be applied on the basis of the diagnosis result.