A device for observing water content contained in a leaf or other part of a plant, the device comprising: a first light source configured to radiate a near infrared laser reference beam of a first wavelength at which light tends not to be absorbed in water while scanning two-dimensionally with respect to the leaf or other part of the plant; a second light source configured to radiate a near infrared laser measuring beam of a second wavelength at which light tends to be absorbed in water while scanning two-dimensionally with respect to the leaf or other part of the plant; an output unit configured to output an invisible light image indicating presence or absence of water contained in the leaf or other part of the plant based on diffuse reflection light of the near infrared laser reference beam reflected on the leaf or other part of the plant and diffuse reflection light of the near infrared laser measuring beam reflected on the leaf or other part of the plant; a water content calculation unit that is configured to repeatedly calculate the water content contained in each pixel area constituting the invisible light image based on the diffuse reflection light of the near infrared laser reference beam and the diffuse reflection light of the near infrared laser measuring beam, in a certain measurement period; and a controller configured to display a time-transition of the water content, which is contained in each pixel area from start to end of the measurement period, calculated by the water content calculation unit on a display unit, wherein the controller is configured to fixedly determine a set of pixel areas in which the water content calculated by the water content calculation unit exceeds a threshold level at the start of the measurement period, out of all pixel areas constituting the invisible light image, as an observation target portion on the leaf or other part of the plant, and the water content calculation unit is configured to calculate the water content contained in the leaf or other part of the plant based on a total sum of a reflection intensity ratio for each pixel area in the observation target portion of the leaf or other part of the plant, 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 at which light tends not to be absorbed in water while scanning two-dimensionally with respect to the leaf or other part of the plant; a second light source configured to radiate a near infrared laser measuring beam of a second wavelength at which light tends to be absorbed in water while scanning two-dimensionally with respect to the leaf or other part of the plant; an output unit configured to output an invisible light image indicating presence or absence of water contained in the leaf or other part of the plant based on diffuse reflection light of the near infrared laser reference beam reflected on the leaf or other part of the plant and diffuse reflection light of the near infrared laser measuring beam reflected on the leaf or other part of the plant; a water content calculation unit that is configured to repeatedly calculate the water content contained in each pixel area constituting the invisible light image based on the diffuse reflection light of the near infrared laser reference beam and the diffuse reflection light of the near infrared laser measuring beam, in a certain measurement period; and a controller configured to display a time-transition of the water content, which is contained in each pixel area from start to end of the measurement period, calculated by the water content calculation unit on a display unit, wherein the controller is configured to fixedly determine a set of pixel areas in which the water content calculated by the water content calculation unit exceeds a threshold level at the start of the measurement period, out of all pixel areas constituting the invisible light image, as an observation target portion on the leaf or other part of the plant, and the water content calculation unit is configured to calculate the water content contained in the leaf or other part of the plant based on a total sum of a reflection intensity ratio for each pixel area in the observation target portion of the leaf or other part of the plant, 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.