When generating a projection image by setting a plurality of search points in an intended display area of a three-dimensional image along a plurality of visual lines, each connecting each pixel on a projection plane on which the three-dimensional image is projected and an arbitrary viewpoint, calculating a pixel value of each search point by an interpolation operation based on pixel values of adjacent pixels of each search point, and determining a pixel value of each pixel on the projection plane with respect to each visual line based on the calculated pixel value of each search point, if an adjacent pixel of a search point is a pixel in an unintended display area, the pixel value of the search point is calculated such that the pixel value of the pixel in the unintended display area does not contribute to the calculation, thereby preventing an artifact arising from the unintended display area.