[Aim] To determine a center of rotation for rotating a projection image that has been projected based on a three-dimensional image to display the rotated projection image on a display screen. [Solution Means] A projection image is generated from a three-dimensional image by projecting an observation target (5) in a predetermined projection direction. The generated projection image is displayed at a center of a display screen of a display device. A region extending in the predetermined projection direction from a central part of the display screen is set as set region (S1), and at least a part of an overlapped region between the set region, which has been set, and the observation target is determined, as rotation center candidate region (CR1). Center (P1) of rotation is determined in the determined rotation center candidate region (CR1) or in the vicinity thereof.
