A device for observing water content in a leaf or part of a plant, the device comprising: a first light source configured to radiate a near infrared laser reference beam of a first wavelength having a characteristic in which light tends not to be absorbed in water while scanning two-dimensionally with respect to the leaf or part of the plant; a second light source configured to radiate a near infrared laser measuring beam of a second wavelength having a characteristic in which light tends to be absorbed in water while scanning two-dimensionally with respect to the leaf or part of the plant; an output unit configured to output an invisible light image of a target portion to be observed of the leaf or part of the plant based on diffuse reflection light of the near infrared laser reference beam reflected on the leaf or part of the plant and diffuse reflection light of the near infrared laser measuring beam reflected on the leaf or part of the plant; a water content calculation unit configured to repeatedly calculate the water content contained in each pixel area constituting the invisible light image based on diffuse reflection light of the near infrared laser reference beam and diffuse reflection light of the near infrared laser measuring beam, in a certain measurement period; and a controller configured to display, as a time-transition of the water content in the target portion from start of the measurement period, a standardized pixel average value of the water content in the target portion and a total sum of the standardized water content in the target portion using the water content contained in the pixel area calculated by the water content calculation unit on a display unit in a comparative manner, wherein the water content calculation unit is configured to calculate water content contained in each pixel area based on a reflection intensity ratio, the reflection intensity ratio being determined as a logarithmized ratio of intensities of the diffuse reflection light of the near infrared laser reference beam and the diffuse reflection light of the near infrared laser measuring beam. a first light source configured to radiate a near infrared laser reference beam of a first wavelength having a characteristic in which light tends not to be absorbed in water while scanning two-dimensionally with respect to the leaf or part of the plant; a second light source configured to radiate a near infrared laser measuring beam of a second wavelength having a characteristic in which light tends to be absorbed in water while scanning two-dimensionally with respect to the leaf or part of the plant; an output unit configured to output an invisible light image of a target portion to be observed of the leaf or part of the plant based on diffuse reflection light of the near infrared laser reference beam reflected on the leaf or part of the plant and diffuse reflection light of the near infrared laser measuring beam reflected on the leaf or part of the plant; a water content calculation unit configured to repeatedly calculate the water content contained in each pixel area constituting the invisible light image based on diffuse reflection light of the near infrared laser reference beam and diffuse reflection light of the near infrared laser measuring beam, in a certain measurement period; and a controller configured to display, as a time-transition of the water content in the target portion from start of the measurement period, a standardized pixel average value of the water content in the target portion and a total sum of the standardized water content in the target portion using the water content contained in the pixel area calculated by the water content calculation unit on a display unit in a comparative manner, wherein the water content calculation unit is configured to calculate water content contained in each pixel area based on a reflection intensity ratio, the reflection intensity ratio being determined as a logarithmized ratio of intensities of the diffuse reflection light of the near infrared laser reference beam and the diffuse reflection light of the near infrared laser measuring beam.