A cardiac cavity region specifying part specifies the position of a cardiac cavity region represented in volume data. An image generation plane determining part determines an image generation plane that includes a rotation axis intersecting the cardiac cavity region. With a direction orthogonal to the image generation plane as a view direction, a first image generator generates three-dimensional image data that three-dimensionally represents a region excluding the cardiac cavity region, based on data excluding data included in the cardiac cavity of the volume data. A second image generator generates two-dimensional image data that two-dimensionally represents a region in the image generation plane, based on the data excluding the data included in the cardiac cavity region of the volume data. An image synthesizer synthesizes the three-dimensional image data with the two-dimensional image data. A display controller causes a display to display the synthesized image.