To provide an image system for remarkably reducing manual optimization for a doctor or a user without exerting a disadvantageous influence by lightness of an image.

In the image system, particularly in an image system for a medical diagnostic device, provided with a circuit for harmonizing image data, and for forming the image to be displayed on a reproducing device by processing image data in the form of a gray value signal, a circuit (8) is configured to automatically intensify contrast by newly correcting a gray value signal and a gray value dynamic range accompanied therewith, while maintaining a C value specifying the center of a gray value range after harmonization that is previously performed.