PURPOSE: To measure an absorption spectrum with a low noise over the entire wavelength range.

CONSTITUTION: Light from a light source 1 is condensed with a condenser lens 2 and introduced to a sample cell 4, undergoes a spectral analysis of the transmission light through a sample with a concave grating 6 via a slit 5 and then, an image of the slit 5 is formed on the surface of a photodiode array 7. On the other hand, reading switches Sn-1, Sn, Sn+1... are controlled to be opened or closed so that the electric load storage time of photodetectors Dn-1, Dn, Dn+1... composing the photodiode array 7 changes according to the intensity of the background. Thus, the storage charge of the photodetectors are kept almost constant without being affected by the intensity of the background.