First beam source radiates a near infrared reference beam of 905 nm, in which light tends not to be absorbed in water toward a leaf of a plant. Second beam source radiates a near infrared measuring beam of 1550 nm, in which light tends to be absorbed in water toward the leaf of the plant. Threshold level setter/water content index detector calculates a water content index of one leaf as a total sum ΣLn(I905/I1550) of the reflection intensity ratio. Controller displays a graph representing a total sum of water content of the leaf and a pixel average value as a time-transition of the water content contained in the plant from the start of the measurement period on a UI screen of a monitor. Viewed from the first and second beam sources, a white reference substrate covering a back surface of the leaf is disposed on the leaf of the plant.