A true leaf and a plurality of artificial leaves are attached on a white reference substrate. A first light source radiates a reference beam of 905 nm while sequentially scanning toward the white reference substrate. A second light source radiates a measuring beam of 1550 nm while sequentially scanning toward the white reference substrate. A threshold level setter/water content index detector calculates a relative water content of a true leaf based on the intensity of each reflection light of the reference beam and the measuring beam which are respectively reflected by the true leaf and a plurality of artificial leaves.