PROBLEM TO BE SOLVED: To provide a method for diagnosing nitrogen deficiency of a plant using an infrared absorption spectrum, which does not need experiment or intuition of a measuring person.SOLUTION: A method for diagnosing nitrogen deficiency in a plant comprises: a step for preparing a water extract of a plant leaf; a step for measuring an infrared absorption spectrum of the water extract; a step for calculating a difference spectrum based on the infrared absorption spectrum of the water extract and an infrared absorption spectrum of the water; a step for correcting a base line of the difference spectrum for obtaining a correction spectrum; and a step for determining that the plant is nitrogen deficiency, when an area ratio expressed by a formula (1) is equal to or more than a reference value in the correction spectrum, in which the area ratio=(a first peak area having a peak top in a wavelength area of 8.3 μm or more and 10.5 μm or less)/(a second peak area having a peak top in a wavelength area of 6.
