1. An immunoassay method for determining the presence or absence of a substance to be detected in a sample by detecting color change due to a specific immunoreaction occurring in each measurement region of a sample analysis tool, the immunoassay method comprising: an optical detection step of detecting the color change in each measurement region of the sample analysis tool; and a determination step of determining the presence or absence of the substance to be detected based on information obtained in the optical detection step, the optical detection step comprising an optical signal measurement step of measuring an optical signal of each measurement region 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 step comprising a discrimination step of 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 step, 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 due to a cause other than the specific immunoreaction, wherein, in the discrimination unit, the comparison criterion is a standard value for a magnitude relationship between the optical signal at the main wavelength and the optical signal at the sub-wavelength, the optical signal at the main wavelength is compared with the optical signal at the sub-wavelength based on the standard value for the magnitude relationship, and the color change due to the specific immunoreaction and the color change due to a cause other than the specific immunoreaction are discriminated based on a result of the comparison, wherein the sample is provided to each measurement region before the optical signal measurement step of measuring an optical signal of each measurement region at each of two or more different wavelengths.