The purpose of this study is to validate a new analysis technique to provide continuous data for cerebral blood flow (CBF) with near infrared spectrophotometer (NIRS, NIRO-500 Hamamatsu Photonics, Japan). In our mathematical model, we obtain two differential equations by considering the in-out balance of the cerebral tissue hemoglobin concentrations in the arterial and venous side. Solving these two equations, we finally obtain the blood flow into the arterial side and out of the venous side separately. To verify the mathematical model, we compared the results of our model analysis with the flow velocity (FV) at the middle cerebral artery, which was measured by transcranial Doppler, in eight volunteers who were exposed to 60 degree head-up tilt. In our results, CBF measured by NIRS was highly correlated to the FV (y equals 1.0003 x + 0.0583, R equals 0.920), where x and y are %flow velocity measured by transcranial Doppler and %CBF measured by NIRS, respectively. Our method provides continuous changes in the cerebral hemodynamics noninvasively in dynamic conditions. It is not necessary to assume that hemodynamics is static during measurements. This method is of use in monitoring the cerebral hemodynamics noninvasively and continuously.