A sixteen-year-old female patient with congenital aortic stenosis underwent Ross procedure. We monitored bilateral regional cerebral saturation of oxygen (rcSO₂) on the forehead at the right and left of the midline. After aortic and bicavel cannulation, cardiopulmonary bypass was instituted. On the mor- row of aortic cross clamping, the rcSO₂ fell from approximately 55% to below 30%. We searched the cause of this phenomenon, and detected that the tip of aortic cannula was inserted to the left subclavian artery. After repositioning, the bilateral rcSO₂ increased to above 65%. We felt keenly that the monitoring of rcSO₂ is useful to recognize corrective adjustment of the cannula ori- entation, and the avoidance of cerebral hypoperfusion during the cardiopulmonary bypass period.