The mental state, such as mood or emotion, of an individual can be apprehended by a method using non-invasive biological light measurement technology. A biological light measurement device, which has an irradiation section, presents different tasks (at least a first task and a second task) to a subject, hemoglobin signals based on changes in the concentration of oxygenated hemoglobin and deoxygenated hemoglobin in the subject are calculated from the strength of light detected by a detection section, and a relative value using the hemoglobin signal at a predetermined measurement channel with respect to the first task, and the hemoglobin signal at a different predetermined measurement channel with respect to the second task is calculated.