A projection image generation apparatus for generating a projection image by processing three-dimensional image data obtained by reconstructing multiple two-dimensional images, the apparatus comprising: a first projection target determination means (11, S2) adapted to determine first three-dimensional image data to be projected from the three-dimensional image data; a first projection image generation means (11, S3) adapted to generate a first projection image by projecting the first three-dimensional image data in a first projection direction by maximum intensity projection or minimum intensity projection; a first coordinate specifying means (11, S4) adapted to specify a first coordinate position in the first projection image; a second projection target determination means (11, S6) adapted to determine second three-dimensional image data to be projected from the three-dimensional image data; a second projection image generation means (11, S7) adapted to generate a second projection image by projecting the second three-dimensional image data in a second projection direction different from the first projection direction; and a second coordinate display means adapted to display a second coordinate position, in the second projection image, obtained by projecting a first voxel in the three-dimensional image data, which corresponds to the pixel at the first coordinate position in the first projection image, onto the second projection image in the second projection direction, and wherein the second projection image generation means (11, S7) is a means that generates a projection image by maximum intensity projection, or by minimum intensity projection, and when the first voxel and a second voxel in the three-dimensional image data corresponding to the pixel at the second coordinate position in the second projection image are different, to indicate that the first voxel and the second voxel are different. a first projection target determination means (11, S2) adapted to determine first three-dimensional image data to be projected from the three-dimensional image data; a first projection image generation means (11, S3) adapted to generate a first projection image by projecting the first three-dimensional image data in a first projection direction by maximum intensity projection or minimum intensity projection; a first coordinate specifying means (11, S4) adapted to specify a first coordinate position in the first projection image; a second projection target determination means (11, S6) adapted to determine second three-dimensional image data to be projected from the three-dimensional image data; a second projection image generation means (11, S7) adapted to generate a second projection image by projecting the second three-dimensional image data in a second projection direction different from the first projection direction; and a second coordinate display means adapted to display a second coordinate position, in the second projection image, obtained by projecting a first voxel in the three-dimensional image data, which corresponds to the pixel at the first coordinate position in the first projection image, onto the second projection image in the second projection direction,