Abstract: A method of and device for analyzing one ingredient in a mixed solution with two light beams of different wavelengths are disclosed which include the following processes. A light beam from a light source are divided into a first light beam having a wavelength of .lambda..sub.1 and a second light beam having a wavelength of .lambda..sub.2. The wavelength .lambda..sub.1 is fixed, and .lambda..sub.2 is variable, for example, a scanning wavelength. Reference signals E.sub..lambda..sbsb.1 and E.sub.(.lambda..sbsb.2.sub.) respectively corresponding to the fixed wavelength .lambda..sub.1 and the variable wavelength (.lambda..sub.2) are produced from photometric signals resulting from the passage of the first and second light beams through only a solvent. After having been normalized by the reference signals E.sub..lambda..sbsb.1 and E.sub.(.lambda..sbsb.2.sub.), the photometric signals Ib.sub..lambda..sbsb.1 and Ib.sub.(.lambda..sbsb.2.sub.