A first beam source radiates, as a reference beam, 905 nm of near infrared beam having a characteristic in which light tends not to be absorbed in water toward a leaf of a plant. A second beam source radiates, as a measuring beam, 1550 nm of near infrared beam having a characteristic in which light tends to be absorbed in water toward the leaf of the plant. A threshold level setter/water content index detector calculates a water content index of one leaf which is a total sum Σ Ln (I905/I1550) of the reflection intensity ratio. A controller displays a graph representing the time-serial change of the water content contained in the plant from the start to the end of the measurement period on a UI screen of a monitor. Further, the controller is connected to an environmental control device that controls environmental conditions surrounding the plant, and instructs the control device to change or maintain the environmental conditions based on a change of increase and decrease in the water content contained in the calculated plant.