A contour of an image is corrected to prevent a dropout in a multicolor print due to registration mismatch. Vector data of two supplemental contours C2a and C2b are obtained from vector data of a contour subjected to the contour correction. The supplemental contours are distanced by a length W from the contour C2. Densities of image parts R1and R2 within the contour correction region R2m that is defined by the supplemental contours are compared with each other, and the higher density (or the lower density) is selected as a correction density for the contour correction region. The contour correction region is filled with the correction density; this is equivalent to shifting the contour C2 to either of the two supplemental contours.