First RGB image signals are subjected to an input process. First color information is obtained from the first RGB image signals. Second color information is obtained by a selective expansion processing in which color ranges except a first color range are moved in a feature space formed by the first color information, the first color information that represents each object in a body cavity being distributed in the each color range. The second color information is converted to second RGB signals. A red display signal, a green display signal and a blue display signal are obtained by applying a pseudo-color display process to the second RGB signals.