An image processing apparatus comprising: a position data specifying unit (19, 21) adapted to specify a plurality of position data ( w K i) indicating a center line position of an input curved tubular observation target in three-dimensional image data of the observation target; a curved cross-section calculating unit adapted to calculate a first curved cross-section ( w x, w y, w z) in a desired line-of-sight direction from said plurality of position data ( w K i) and line-of-sight data ( s T w; w T s) including the line-of-sight direction; a projection image generating unit adapted to generate a projection image by projecting, along the line-of-sight direction, the three-dimensional image data on the first curved cross-section onto a projection surface or by projecting, along the line-of-sight direction, three-dimensional image data concerning a volume including the first curved cross-section and having a predetermined thickness onto a projection surface; and a display unit adapted to display the projection image, wherein the curved cross-section calculating unit is adapted to calculate an intermediate curved cross-section ( s x, s y, s z) in a screen coordinate system by transforming said plurality of position data set ( w K i) on the three-dimensional image data into position data ( s K i) in the screen coordinate system according to the line-of-sight direction, and using a weight with a coefficient (δ i), which changes in accordance with distances (D i) from said plurality of position data ( s K i), to determine depth information ( s z i) along the line-of-sight direction of each point on the screen ( s P x,y) in the screen coordinate system, and to calculate the first curved cross-section in the line-of-sight direction by inversely transforming the intermediate curved cross-section ( s x, s y, s z) into a three-dimensional space. a position data specifying unit (19, 21) adapted to specify a plurality of position data ( w K i) indicating a center line position of an input curved tubular observation target in three-dimensional image data of the observation target; a curved cross-section calculating unit adapted to calculate a first curved cross-section ( w x, w y, w z) in a desired line-of-sight direction from said plurality of position data ( w K i) and line-of-sight data ( s T w; w T s) including the line-of-sight direction; a projection image generating unit adapted to generate a projection image by projecting, along the line-of-sight direction, the three-dimensional image data on the first curved cross-section onto a projection surface or by projecting, along the line-of-sight direction, three-dimensional image data concerning a volume including the first curved cross-section and having a predetermined thickness onto a projection surface; and a display unit adapted to display the projection image, wherein the curved cross-section calculating unit is adapted to calculate an intermediate curved cross-section ( s x, s y, s z) in a screen coordinate system by transforming said plurality of position data set ( w K i) on the three-dimensional image data into position data ( s K i) in the screen coordinate system according to the line-of-sight direction, and using a weight with a coefficient (δ i), which changes in accordance with distances (D i) from said plurality of position data ( s K i), to determine depth information ( s z i) along the line-of-sight direction of each point on the screen ( s P x,y) in the screen coordinate system, and to calculate the first curved cross-section in the line-of-sight direction by inversely transforming the intermediate curved cross-section ( s x, s y, s z) into a three-dimensional space.