There is provided a plant stress detection apparatus including a first light source that radiates a reference beam of a first wavelength that has a characteristic of tending not to be absorbed in water toward a plant by optical scanning, a second light source that radiates a measuring beam of a second wavelength that has a characteristic of tending to be absorbed in water toward the plant by optical scanning, and a detector that detects presence or absence of water or undulation of the plant based on reflection light of the reference beam that is reflected at an irradiation position of the plant that has a first irradiation area and reflection light of the measuring beam that is reflected at an irradiation position of the plant that has a second irradiation area that is different from the first irradiation area.
