To provide a hydroponic method of a plant, comprising effectively utilizing a waste nutritious liquid of hydroponics and carrying out a cleaning treatment.

The hydroponic method of the plant comprises: collecting the waste nutritious liquid discharged from a hydroponic site 2 of the plant in a waste nutritious liquid-storing tank 3; flowing the waste nutritious liquid at a fixed quantity to a culture vessel 4 of conferva; culturing the conferva to clean the resultant waste nutritious liquid; harvesting the conferva obtained by the culture; adding water thereto; extracting the alga body component from the resultant mixture by a hot-water extraction device 5; taking out the extract containing the alga body component from a storing vessel 6 of the extract; diluting the extract with water in an extract-diluting vessel 7; and spraying the diluted product on the surface of a leaf of the plant cultivated in the hydroponic site 2 of the plant through a pump 10.