When a three-dimensional image is to be displayed, wall coordinate data on the three-dimensional image is created based on echo data (S21), and the wall coordinate data is subjected to a conversion such as a movement, rotation, or the like in correspondence with the current state (S22). Furthermore, an intersecting portion with a three-dimensionally displayed cubic portion in the wall coordinate data is extracted, and an image of the wall surface is rendered based on the data on the intersecting portion (S23). Then, a portion intersecting with the wall coordinate data after the conversion processing is removed on each surface of the cubic, and a tomogram corresponding to each surface is rendered in the remaining region (S24), so that the three-dimensional image is rendered (S25). If there is an operation input by an image input device, the process is returned to step 22, and the processing corresponding to the input operation is repeated. As a result, a three-dimensional image in the display state corresponding to the operation input is rendered in real time. Thus, it becomes possible to change and display the display state of a display object with a simple operation.
