PURPOSE: To make manual control unnecessary by varying the shading level of the image of a body to be diagnosed so as to be optimum in real time, based on parameters for displaying the image at the optimum shading level which is determined on the basis of the prescribed level of the gradation data in the data for diagnosis which is at a prescribed level.

CONSTITUTION: A high-speed memory controller 3 reads out sequentially picture element data stored in a display memory 2, detecting the gradation data at the maximum level in the picture cells thereof and delivering them to an arithmetic device 4. When the maximum levels of the input gradation data are a1 and a2, for instance, the arithmetic device 4 determines specified level conversion tables f(a1) and f(a2) out of a number of level conversion tables stored, and delivers said tables f(a1) and f(a2) to a display controller 5. The display controller 5 converts the gradation data in the picture element data based on the level conversion tables input from the arithmetic device 4, and displays the image of a body to be diagnosed on the picture plane of a CRT monitor 6 at the optimum shading level and in real time.