An infrared spectrometer disperses a radiation ray from a catalyst which has been exposed to a gas for evaluation to adsorb adsorbates thereon, and outputs spectral data in accordance with a wavenumber of the radiation ray to a computer storing reference data. The computer normalizes the spectral data and the reference data, and then, it calculates a product of the normalized spectral data and the reference data. Thereafter, a function of the product is differentiated with respect to the wavenumber to obtain a differential function. Accordingly, a specified wavenumber for which the differential function is zero is determined, so that the common peak of the spectral data and the reference data at the specified wavenumber is accurately determined.