PURPOSE: To easily detect even a fine change in a non-destroying manner, by a method wherein an after-manufacture performance of an organic photosensitive material is measured by a Fourier transformation infrared acoustic spectral method, and a difference spectrum between an object to be measured and a standard sample is computed and processed by a computer with built-in spectrophotometer to find it.

CONSTITUTION: Using a photo acoustic spectral method by a Fourier transformation infrared spectrophotometer as a method to detect, before the use, a chemical change during conservation of a photosensitive layer of an organic photosensitive material such as photosensitive printing plates, photoresistors for IC, a measurement is easily and precisely conducted in a non-destroying manner to prevent the production of a poor product. For example, a reduction in bands 2,160, 2,120cm-1 of a diazo group of o-napthoquinonediazido and an increase in 1,720cm-1 produced by a photo chemical reaction are easily and high-precisley measured by this method, and thereby the measurement of increase and decrease in said three bands of two samples of a standard sample and a deteriorating sample permits verification before using.