Modern people are exposed to various diseases by various causes. As a result, interest in health continues to increase and regular health checkups become mandatory. Thus, facilities for measuring disease are becoming more diverse and complex. Near-infrared spectroscopy is harmless to human body and equipment and inspection cost are low. In addition, because of the advantages such as simple and quick measurement time, development of the equipment for measuring brain diseases is proceeding most actively. In this study, cerebral blood flow signal changes of mental stress were measured by near infrared spectroscopy. The study was conducted by 10 men and women in their 20s and 30s who were physically fit. The changes in the blood flow signal of the first ballast and mental deficit were significantly different from the confidence level of 5% to P <0.001. Mental stress causes physical changes such as heart rate variability, electrical skin reactions, and blood flow changes. Future research needs to be carried out in various directions of daily life such as stress, arousal, sleeping, fatigue state in order to develop NIR spectroscopy equipment.