Projection is performed in such a manner that the same point on a moving internal organ can be constantly observed, thereby improving the accuracy of diagnosis. Motion information about the motion of a projection target between one set of three-dimensional volume data in a group of sets of three-dimensional volume data and a different set of three-dimensional volume data in the group of sets of three-dimensional volume data that was obtained by imaging at a different time from the time of imaging of the one set of three-dimensional volume data is obtained based on the projection target that is present in the one set of three-dimensional volume data and the projection target that is present in the different set of three-dimensional volume data. Then, the viewpoint of projection is moved to a position at which the projection target in the different set of three-dimensional volume data can be projected.