A storage unit stores a plurality of vectors and a plurality of past electrocardiographic phases in association with each other. Each of the vectors is a vector from a past reference point to a past objective point. A Reference point specifying unit specifies a current reference point on a current image. An electrocardiograph detects a current electrocardiographic phase associated with the current image. A vector specifying unit specifies a specific vector associated with a past electrocardiographic phase corresponding to the detected current electrocardiographic phase among the plurality of vectors. Objective point calculation unit calculates a position of a current objective point on the current image based on the specified vector and the position of the current reference point. A display unit displays the position of the current objective point on the current image.