In a blood information acquisition mode for obtaining an oxygen saturation level of hemoglobin in a blood vessel, preliminary imaging and main imaging are performed. In the preliminary imaging, a normal internal body part is imaged. A blood information calculation section calculates an oxygen saturation level of each pixel. A changing section corrects standard reference data in accordance with a difference between an average of the oxygen saturation levels obtained in the preliminary imaging and a predetermined standard value of the oxygen saturation level. In the subsequent main imaging, corrected reference data is used to calculate an oxygen saturation level of each pixel corresponding to an internal body part being observed.