A method for generating a projection image from three-dimensional image data, including the steps of determining first three-dimensional image data to be projected from the three-dimensional image data, generating 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, specifying a first coordinate position in the first projection image, determining second three-dimensional image data to be projected from the three-dimensional image data, generating a second projection image by projecting the second three-dimensional image data in a second projection direction different from the first projection direction, and displaying 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 from the second projection direction.