In a special mode, an internal body portion is imaged under a first white light beam to obtain a first frame composed of blue, green, and red signals. Sequentially, the body portion is imaged under a second white light beam to obtain a second frame composed of blue, green, and red signals. An oxygen saturation level is calculated from the signals. A special image is produced based on the oxygen saturation level. A displacement between the first and second frames is calculated from the signals. If the displacement is a first allowable value or more and less than a second allowable value, the special image is displayed with a lower chroma on a monitor, as compared with a case where the displacement is less than the first allowable value. If the displacement is the second allowable value or more, the special image is converted into a gray scale image.
