A method for designating a region of interest from volume data of a subject. When a user clicks a mouse to specify designation points, a CPU generates a curved surface including the designation points. The CPU executes a hidden surface removal process on the curved surface to generate a plotted region, sets a thickness for the plotted region, and determines the region of interest. The CPU performs MIP processing on the region of interest and a region of interest re-designating process on the MIP image. In the region of interest re-designating process, the user clicks the mouse on the MIP image, and the CPU determines the MIP position of the clicked position. The CPU sets the MIP position as a new designation point, and re-specifies a new region of interest by performing a region of interest designating process based on the new designation point.