In a special mode, a superficial wavelength set having plural types of narrow band light in a blue wavelength band of 400 to 500 nm is chosen. The plural types of narrow band light are successively applied to an internal body portion. A CCD captures images of the internal body portion under the narrow band light. A blood information calculation section calculates an oxygen saturation level of hemoglobin in a blood vessel based on an image signal. A comparison section compares the calculated oxygen saturation level with a predetermined threshold value. When the oxygen saturation level is less than the threshold value, a hypoxic region detection signal is outputted to a wavelength set switching section. The wavelength set switching section switches from the superficial wavelength set to a middle wavelength set and to a deep wavelength set, so the oxygen saturation levels at middle and deep depths are measured.