A device for observing water content contained 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 sequentially scanning the leaf or part of the plant; a second light source configured to radiate an infrared laser measuring beam of a second wavelength having a characteristic in which light tends to be absorbed in water while sequentially scanning the leaf or part of the plant; an image sensor which has a peak of spectral sensitivity with respect to wavelengths of both of the near infrared laser reference beam and the infrared laser measuring beam; a water content calculation unit that is configured to repeatedly calculate the water content contained in the leaf or part of the plant based on an image of diffuse reflection light of the near infrared laser reference beam reflected on the leaf or part of the plant and an image of diffuse reflection light of the infrared laser measuring beam reflected on the leaf or part of the plant captured by the image sensor, in a certain measurement period; a controller that is configured to display a time-serial change of the water content, which is contained in the leaf or part of the plant from start to end of a measurement period, calculated by the water content calculation unit on a display unit; a detection unit that is configured to detect positional deviation of the leaf or part of the plant by comparing a difference of the calculated water content contained in the leaf or part of the plant before and after a designated elapsed time with a threshold level set in advance, wherein a positional deviation is determined in case the difference of the water content contained in the leaf or part of the plant exceeds the threshold level; and a coefficient calculation unit that is configured to calculate a correction coefficient with respect to a water content after positional deviation based on the calculated water content before and after the positional deviation in a case where the positional deviation of the leaf or part of the plant is detected, wherein the controller is configured to correct the calculated water content after the positional deviation using the correction coefficient calculated by the coefficient calculation unit and display a correction result on the display unit. 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 sequentially scanning the leaf or part of the plant; a second light source configured to radiate an infrared laser measuring beam of a second wavelength having a characteristic in which light tends to be absorbed in water while sequentially scanning the leaf or part of the plant; an image sensor which has a peak of spectral sensitivity with respect to wavelengths of both of the near infrared laser reference beam and the infrared laser measuring beam; a water content calculation unit that is configured to repeatedly calculate the water content contained in the leaf or part of the plant based on an image of diffuse reflection light of the near infrared laser reference beam reflected on the leaf or part of the plant and an image of diffuse reflection light of the infrared laser measuring beam reflected on the leaf or part of the plant captured by the image sensor, in a certain measurement period; a controller that is configured to display a time-serial change of the water content, which is contained in the leaf or part of the plant from start to end of a measurement period, calculated by the water content calculation unit on a display unit; a detection unit that is configured to detect positional deviation of the leaf or part of the plant by comparing a difference of the calculated water content contained in the leaf or part of the plant before and after a designated elapsed time with a threshold level set in advance, wherein a positional deviation is determined in case the difference of the water content contained in the leaf or part of the plant exceeds the threshold level; and a coefficient calculation unit that is configured to calculate a correction coefficient with respect to a water content after positional deviation based on the calculated water content before and after the positional deviation in a case where the positional deviation of the leaf or part of the plant is detected, wherein the controller is configured to correct the calculated water content after the positional deviation using the correction coefficient calculated by the coefficient calculation unit and display a correction result on the display unit.