A CPU executes a gray value gradient calculation process to, for example, an R signal in step S10, executes an isotropic change feature value calculation process based on a gray value gradient calculated in step S11, and further executes a possible polyp detection process for generating a possible polyp image at a location where a polyp exists based on an isotropic change feature value calculated in step S12. This improves the detection of an existing location of an intraluminal abnormal tissue.