Regional cerebral oxygen saturation (rSO2) and the bispectral index (BIS) are used to detect cerebral perfusion abnormalities. However, whether rSO2 and BIS values change during ligation of the internal jugular vein (IJV) is unknown. We report a case in which BIS values were decreased, despite increased rSO2 during ligation of the IJV. A 72-year-old man was diagnosed with metastasis of renal cancer to the thyroid associated with tumor embolism in the right IJV. Thyroidectomy with total laryngectomy was performed. After right IJV ligation, right rSO2 was increased from 73 to 78 %, while the right BIS value was decreased from 40 to 27. Contralateral rSO2 and BIS values were unchanged. Right rSO2 and BIS values returned to pre-ligation values in 10 min. Ligation of the IJV might increase cerebral blood flow and ipsilateral rSO2. Physicians should use BIS values with caution during IJV ligation because a sudden decrease in the BIS value is not always associated with cerebral hypoperfusion.