To provide an analyzer capable of accurately judging that the lowering of the quantity of passing light received by a light receiving system is caused by the deterioration of a light source, and a deterioration detection method of the light source.

The deterioration detecting part 34 of the light source in the analyzer 1 of this invention does not use the absolute value of the quantity of the passing light of a blank sample but uses a change in the relative value on a long wavelength side in the quantity of the passing light of the blank sample and a change in the relative value on a short wavelength side in the quantity of the passing light of the blank sample to judge the presence of the deterioration of the light source on the basis of the wavelength distribution dependence of the intensity of light at each of color temperatures. Accordingly, it is accurately judged that the lowering of the quantity of the passing light received by the light receiving system is caused by the deterioration of the light source.