In the brain activation measurements by near infrared spectroscopy (NIRS), the partial optical path length in the brain, which is an index of the sensitivity of the NIRS signal to the brain activation, is strongly affected by the thickness and the structure of the superficial tissues. In this study, we investigate the influence of the frontal sinus on the NIRS signal of the brain activation. The light propagation in a simplified head model including a void region mimicking the frontal sinus is predicted by Monte Carlo simulation to investigate the influence of the frontal sinus on the partial optical path length in the brain and the mean optical path length in the head. The frontal sinus strongly affects the light propagation in the head. The partial optical path length for small source-detector separation tends to be increased by the presence of the frontal sinus whereas that for large source-detector separation is decreased by the influence of the frontal sinus.