PURPOSE: To control a culture solution fed to a plant to the optimum state by efficiently performing each control for the electric conductivity, pH value or various ion concentrations in the culture solution.

CONSTITUTION: The electric conductivity, pH value and various ion concentrations of a culture solution (2) are measured with a sensor part (10) and control is performed with a controller (20) so as to respectively make the electric conductivity, pH value and various ion concentrations coincide with target ranges according to the priority order. When an epistatic item, e.g. the electric conductivity deviates from the target range during the control of a hypostatic item of the priority order, e.g. the ion concentration, the control of the ion concentrations is stopped to preferentially control the electric conductivity to this state.