A device for observing water content which is disposed facing a background material (bd) which covers back surfaces of an observation portion of a plant and at least one sample used as a reference for deriving water content of the observation portion of the plant, the device comprising: a first light source configured to radiate a reference beam having a characteristic in which light tends not to be absorbed in water while sequentially scanning toward the background material (bd); a second light source configured to radiate a measuring beam having a characteristic in which light tends to be absorbed in water while sequentially scanning toward the background material (bd); wherein the sample used as a reference comprises a component having absorption spectra equivalent to water absorption spectra for each wavelength of the reference beam and the measuring beam; the device further comprising: a water content derivation unit that is configured to derive water content contained in the observation portion based on each reflection light of the reference beams reflected on the observation portion and the sample and each reflection light of measuring beams reflected on the observation portion and the sample, in a certain measurement period; and a controller that is configured to display a time-serial change of the water content, which is contained in the observation portion during the measurement period, derived by the water content derivation unit on a display unit; wherein the water content derivation unit is configured to derive the water content contained in the observation portion of the plant based on a total sum of a reflection intensity ratio in all pixels in one frame image of the observation portion of the plant, the reflection intensity ratio being determined as a logarithmized ratio of intensities of the reflection light of the reference beam and the reflection light of the measurement beam. a first light source configured to radiate a reference beam having a characteristic in which light tends not to be absorbed in water while sequentially scanning toward the background material (bd); a second light source configured to radiate a measuring beam having a characteristic in which light tends to be absorbed in water while sequentially scanning toward the background material (bd); wherein the sample used as a reference comprises a component having absorption spectra equivalent to water absorption spectra for each wavelength of the reference beam and the measuring beam; the device further comprising: a water content derivation unit that is configured to derive water content contained in the observation portion based on each reflection light of the reference beams reflected on the observation portion and the sample and each reflection light of measuring beams reflected on the observation portion and the sample, in a certain measurement period; and a controller that is configured to display a time-serial change of the water content, which is contained in the observation portion during the measurement period, derived by the water content derivation unit on a display unit; wherein the water content derivation unit is configured to derive the water content contained in the observation portion of the plant based on a total sum of a reflection intensity ratio in all pixels in one frame image of the observation portion of the plant, the reflection intensity ratio being determined as a logarithmized ratio of intensities of the reflection light of the reference beam and the reflection light of the measurement beam. a water content derivation unit that is configured to derive water content contained in the observation portion based on each reflection light of the reference beams reflected on the observation portion and the sample and each reflection light of measuring beams reflected on the observation portion and the sample, in a certain measurement period; and a controller that is configured to display a time-serial change of the water content, which is contained in the observation portion during the measurement period, derived by the water content derivation unit on a display unit; wherein the water content derivation unit is configured to derive the water content contained in the observation portion of the plant based on a total sum of a reflection intensity ratio in all pixels in one frame image of the observation portion of the plant, the reflection intensity ratio being determined as a logarithmized ratio of intensities of the reflection light of the reference beam and the reflection light of the measurement beam.