1. An electronic endoscope system comprising: an electronic endoscope that can be inserted into a test subject to capture images of the interior of the test subject; a changed-area detecting device for detecting a changed area in an image captured by said electronic endoscope using data of the captured image, the changed area having different features from other area of the captured image and corresponding to an artificial object; a mask data producing device for producing 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; and an image processing device for processing the captured image according to the mask data, wherein a region containing the artificial object is not enhanced while a remainder of the image is enhanced, and wherein said changed-area detecting device detects the changed pixels in a designated partial area of the captured image, combines the changed pixels into clusters, determines among these clusters such clusters that are equal to or greater than a predetermined size to be pre-changed areas, detects those pixels having the same color as a representative color of the pre-changed areas outside the pre-changed areas to combine with the pre-changed areas into clusters, and determines among these clusters such a cluster that is equal to or greater than the predetermined size to be the changed area.