An irreversible-reaction measurement method comprising: a step in which a perturbation is applied to one of the divided portions of a measurement sample placed in a light path of a Fourier-transform spectrophotometer to cause an irreversible-reaction while a mirror of the spectrophotometer remains at a data point; a step in which interferogram is detected from the sample portion placed in the path at predetermined time intervals after the application of the perturbation; a step in which the mirror moves to and remains at the next data point after the reaction of the sample portion reaches an end point; a step in which the sample portion placed in the light path is changed to the next sample portion each time the mirror moves to the next data point; and a step in which the irreversible-reaction of the measurement sample is analyzed in accordance with the interferogram obtained by repeating the steps.