An electronic endoscope system includes a light source device for sequentially emitting kinds of light having different wavelength bands, an electronic endoscope for sequentially illuminating the kinds of light to a subject tissue containing blood vessels, receiving the kinds of reflected light, and outputting image data corresponding to the kinds of received light, an extractor for extracting positions of a blood vessel from each image data corresponding to the kinds of light, an aligner for aligning images corresponding to the image data based on the positions, an image producer for producing an oxygen saturation level image representing the distribution of the oxygen saturation level in the blood vessel from the image data of the aligned images, and an image monitor for displaying the oxygen saturation level image in simulated color.
