To display a brownish area distinguishably from residuum, a B image signal having a blue narrow-band signal corresponding to blue narrow-band light and a G image signal, out of a color image signal obtained by an endoscope (12), are inputted to a processor device (16). The B image signal is assigned as a B display image signal. The B display image signal and the G image signal are weighted independently, and the sum of the weighted B display image signal and the weighted G image signal is assigned as a G display image signal. The G image signal is assigned as an R display image signal. Based on the RGB display image signals, a special light image is displayed on a monitor (18). In the special light image, the residuum or residual fluid (80) is displayed in a yellowish color, and structure including blood vessels (82) and a lesion is visible translucently under the residuum (80).
