A high accuracy method for detecting a biological-related substance is provided by which an abnormal state, such as the adhesion of dust, the reduction of a sample solution, or the like can be judged. Means for detecting a light emitted by the portion for light detection after dividing the light into at least a plurality of wavelength zones is provided. One of the plurality of wavelength zones includes substantially the same wavelength zone as that of the component of the excitation light. The light intensity of the component of the excitation light is detected and it is compared with a predetermined intensity (threshold). A highly accurate fluorescence measurement can be realized, by which the abnormality of a sample can be judged.