In this study, we measured brain activity using near-infrared spectroscopy (NIRS) when a person was feeling discomfort caused by vibrations. We used the variance in oxygenated hemoglobin (oxy-Hb) levels as an evaluation index. Correlation coefficients were derived from the results of brain function measurements and sensibility evaluation of discomfort using a questionnaire. As a result, a high negative correlation was observed between discomfort and both vibration and brain activation around the medial prefrontal cortex, and a high positive correlation was observed between discomfort and both vibration and brain activation around the lateral prefrontal cortex. This suggests the possibility of evaluating discomfort on the basis of brain activation.