In an oxygen saturation level measurement mode, a color image sensor images an internal body portion alternately irradiated with measurement light and normal light. A normal image is produced from image data obtained under irradiation with the normal light. An oxygen saturation level is calculated from image data obtained under irradiation with the measurement light. Based on the oxygen saturation level, a hyperoxic region and a hypoxic region are determined in the normal image. A color balance process and a color enhancement process are applied to the hyperoxic region to improve visibility of depressions and projections of internal body tissue and a blood vessel pattern. A gain process is applied to the hypoxic region to make distinct color variations according to the degree of the oxygen saturation level.