A projection image of a three dimensional image is generated and displayed and a three dimensional target point corresponding to a designated two-dimensional target point is set to the three dimensional image. A reference cross-section is set to the three dimensional image by using the three dimensional target point. A cross-sectional two-dimensional image of the reference cross-section in which an object close to a point of view that faces the reference cross-section is not displayed is generated. An instruction to change the position of the point of view, using the three dimensional target point based on the cross-sectional two-dimensional image as a reference point, in order to align an observation direction of a structure to which the three dimensional target point is set with a target three dimensional direction is received. A new reference cross-section is set. A new cross-sectional two-dimensional image of the new reference cross-section is generated.