1. An immunoassay analyzer for determining the presence or absence of a substance to be detected in a sample by detecting color change due to a specific immunoreaction in a sample analysis tool, the immunoassay analyzer comprising: an optical detection unit for detecting color change in the sample analysis tool; and a determination unit for determining the presence or absence of the substance to be detected based on information from the optical detection unit, the optical detection unit comprising an optical signal measurement unit for measuring an optical signal at each of two or more different wavelengths including a main wavelength for detecting the color change due to the specific immunoreaction and a sub-wavelength other than the main wavelength, the determination unit comprising a discrimination unit for comparing the respective optical signals at the two or more different wavelengths, including the optical signal at the main wavelength and discriminating between the color change due to the specific immunoreaction and color change due to a cause other than the specific immunoreaction based on a comparison criterion determined previously, wherein, in the determination unit, when no color change in the sample analysis tool is detected, it is determined that the sample is negative regarding that the substance to be detected is not present, when color change in the sample analysis tool is detected, it is determined that the sample is positive regarding that the substance to be detected is present in the case where the discrimination unit determines that the detected color change is due to the specific immunoreaction, and it is determined that the determination is invalid in the case where the discrimination unit determines that the detected color change is clue to a cause other than the specific immunoreaction,