In an electronic endoscope system, a changed-area detector detects a changed area from an image captured by an endoscope, the changed area having different features from other area of the captured image. A mask data produce produces mask data based on the detected changed area, the mask data allocating an image processing parameter to each pixel of the captured image such that the changed area is processed in a different way from the other area. An image processor processes the captured image according to the mask data. Thus, an image area corresponding to an artificial object like a surgical tool may be detected as the changed area and excluded from unnecessary image processing.