In a water content evaluation method, when water content of a part of a plant is evaluated, a white reference substrate (background material) is disposed so as to cover a back surface of a leaf of plant. A first beam source radiates a near infrared beam (reference beam) with a wavelength of 905 nm that has a characteristic of tending not to be absorbed in water toward leaf. A second beam source radiates a near infrared beam (measuring beam) with a wavelength of 1550 nm that has a characteristic of tending to be absorbed in water toward the leaf. Threshold level setter/water content index detector calculates a water content index of one leaf that is total sum Σ Ln (I905/I1550) of the reflection intensity rate at all irradiation positions of the leaf based on a reflection light of a reference beam and a reflection light of a measuring beam that are reflected on all irradiation positions of the leaf.