This invention provides a plant health diagnostic method and a plant health diagnostic device for detecting initial disorder of a plant individual body carrying pests or the like in a large-scale plant cultivation field. The plant health diagnostic method is for diagnosing a health state of a plant in accordance with chlorophyll fluorescence emitted from chlorophyll of a plant body. The method includes: obtaining a first minimum point s after a maximum point p having largest chlorophyll fluorescence intensity and a first maximum point m after the minimum point s on a time course curve for the chlorophyll fluorescence intensity; defining chlorophyll fluorescence intensity values at the minimum point s and the first maximum point m after the minimum point s as S and M, respectively; and comparing the value S and the value M to evaluate the state of the plant. The state of the plant, which cannot be visually recognized from outer appearance, can be detected by comparison between the chlorophyll fluorescence inte
