An image processing device for an endoscope system, comprising: an image input section (48; 206) for inputting a first image, the first image including a first structure brighter than a mucous membrane and a second structure darker than the mucous membrane; and a suppression and reversal section (65, 78, 79) for performing a suppression process and a tone reversal process on the first image to produce a second image, a display of the second structure being suppressed in the suppression process, the first structure being darker than the mucous membrane in the second image, and the mucous membrane having substantially the same brightness as the second structure in the second image, wherein the suppression and reversal section (65, 78, 79) extracts the first structure and the second structure from at least one channel of three RGB channels which constitute the first image for generating a structure-extracted image signal in which a signal value is different between the first structure and the second structure in a positive or negative direction, respectively, with respect to the signal value of the mucous membrane, and converts a negative input value of the structure-extracted image signal to a positive output value and converts a positive input value of the structure-extracted image signal to a positive or zero output value for generating a pre-suppression image, and adds the pre-suppression image to said at least one channel of three RGB channels which constitute the first image for generating a second structure-suppressed image, and reverses gradation of RGB image data of the second structure-suppressed image for generating RGB image data of the second image, and wherein the first structure is a ductal structure and the second structure is capillary vessels. an image input section (48; 206) for inputting a first image, the first image including a first structure brighter than a mucous membrane and a second structure darker than the mucous membrane; and a suppression and reversal section (65, 78, 79) for performing a suppression process and a tone reversal process on the first image to produce a second image, a display of the second structure being suppressed in the suppression process, the first structure being darker than the mucous membrane in the second image, and the mucous membrane having substantially the same brightness as the second structure in the second image, wherein the suppression and reversal section (65, 78, 79) extracts the first structure and the second structure from at least one channel of three RGB channels which constitute the first image for generating a structure-extracted image signal in which a signal value is different between the first structure and the second structure in a positive or negative direction, respectively, with respect to the signal value of the mucous membrane, and converts a negative input value of the structure-extracted image signal to a positive output value and converts a positive input value of the structure-extracted image signal to a positive or zero output value for generating a pre-suppression image, and adds the pre-suppression image to said at least one channel of three RGB channels which constitute the first image for generating a second structure-suppressed image, and reverses gradation of RGB image data of the second structure-suppressed image for generating RGB image data of the second image, and wherein the first structure is a ductal structure and the second structure is capillary vessels.