A threshold level setter/water content index detector calculates an ΣLn (I905/I1550) which is a total sum of the reflection intensity ratio as a water content index of one leaf. A controller displays a graph representing the time-transition of the water content contained in the leaf of the plant from the start to the end of the measurement period on a UI screen of monitor 50. The controller fixedly determines and sets, as a leaf, a set of reflection positions where the water content for each reflection position which is calculated at the start of the measurement period exceeds a threshold level. When viewed from first beam source and second beam source, white reference substrate which covers a back surface of the leaf of the plant is disposed on the leaf of the plant.
