Based on medical image data acquired in each time phase included in a one-cycle interval, first tracking part tracks the position of a first region of interest set in a first time phase in each time phase and tracks the position of a second region of interest set in a second time phase in each time phase. Based on position information of the first region of interest and the second region of interest in each time phase, position correction part obtains position information of a region of interest in each time phase so that it passes through the position of the first region of interest in the first time phase and the position of the second region of interest in the second time phase. Motion-information calculator obtains motion information of a tissue based on the obtained position information.
