An endoscope image input unit 60 receives a current endoscope image signal that is output from an endoscope, which is currently inserted into a subject, and is used to calculate the oxygen saturation. A spectral estimation section 70 generates a spectral estimation image by performing spectral estimation processing on a past endoscope image signal that is obtained during the past endoscope insertion and is different from a signal for oxygen saturation calculation. An oxygen saturation calculation section 74 calculates the current oxygen saturation based on the current endoscope image signal, and calculates the past oxygen saturation based on the spectral estimation image. An oxygen saturation image generation section 80 generates a current oxygen saturation image based on the current oxygen saturation, and generates a past oxygen saturation image based on the past oxygen saturation. A monitor 18 displays the current oxygen saturation image and the past oxygen saturation image.