A first image is projective transformed into a reference image by using a projective transformation matrix on each pixel of the first image. A projective transformation matrix for transforming a tilted image n into a central image G0 is computed based on positions of combinations of twelve corresponding points indicated in a combination of the central image G0 with the tilted image n. The computed projective transformation matrix is then employed to perform a projective transformation of the tilted image n. A projective-transformed tilted image n1 is created thereby.