PROBLEM TO BE SOLVED: To quantitatively and time-sequentially present the transition of a water content in a plant and to accurately capture the transition of the water content from the initial time with respect to the degree of imparted water stress to the plant.SOLUTION: The threshold setting/water index detection processing part 27a calculates ΣLn (I/I) which is the sum of reflection intensity ratios, as the water index of one leaf. A control unit 11 displays on a UI screen 60 of a monitor 50 a graph showing the transition of the time-sequential water content in the leaves of a plant from the start to the end of a measurement period. The control unit 11 fixedly determines and sets as a leaf PT3 a gathering of reflection positions where the water content for each reflection position calculated at the start of the measurement period exceeds a threshold value Sh. On the leaf of the plant, a white background plate covering the back of the leaf of the plant as viewed from the first projection light source and the second projection light source is arranged.SELECTED DRAWING: Figure 28【課題】植物に含まれる水分量の推移を定量的かつ時系列に提示し、植物に対する水ストレスの付与の程度に関して初期時からの水分量の推移を正確に捉える。【解決手段】閾値設定／水分指数検出処理部２７ａは、反射強度比の総和であるΣＬｎ（Ｉ９０５／Ｉ１５５０）を、葉１枚の水分指数として算出する。制御部１１は、測定期間の開始時から終了時までの植物の葉に含まれる水分量の時系列の推移を表すグラフをモニタ５０のＵＩ画面６０に表示する。制御部１１は、測定期間の開始時に算出した反射位置毎の水分量が閾値Ｓｈを超えた反射位置の集合を、葉ＰＴ３として固定的に決定して設定する。植物の葉には、第１の投射光源及び第２の投射光源から見て、植物の葉の背面を覆う白色背景板が配置される。【選択図】図２８
