A plurality of position data are input, which indicate the position of an observation target defined in a three-dimensional space, on its three-dimensional image, which is viewed from a predetermined view point along a predetermined line-of-sight direction. A curved cross-section in the line-of-sight direction is calculated from the plurality of position data and line-of-sight data including the predetermined view point and the predetermined line-of-sight direction. A projection image is generated by projecting the three-dimensional image on the calculated curved cross-section onto a projection surface along the line-of-sight direction. The projection image is then displayed.