A storage unit stores first and second calibration curves. The first calibration curve represents a relationship between an absorbance of a first standard sample for light at a wavelength λ1 and a concentration of a target composition in the first standard sample. The second calibration curve represents a relationship between an absorbance of a second standard sample for light at a wavelength λ2 and a concentration of the target composition in the second standard sample. The second standard sample has a higher concentration than that of the first standard sample. When the absorbance of the unknown sample for the light at the wavelength λ1 is less than a threshold, a concentration measurement processing unit measures a concentration of the target composition in the unknown sample, based on the absorbance of the unknown sample for the light at the wavelength λ1 and the first calibration curve.