A filter coefficient calculating unit 81 sets as a filter window a range, which contains plural coordinates including an attention coordinate “p” in an initial alpha map, and uses a pixel value Ip at a coordinate of an original image corresponding to a coordinate “p” in the filter window and a pixel value Iq at a coordinate of the original image corresponding to a coordinate “q” in the filter window to calculate a filter coefficient Kq at the coordinate “q” on a coordinate to coordinate basis in the filter window. That is, a filter coefficient set is created, which consists of filter coefficients calculated on a coordinate to coordinate basis in the filter window. A weighted-average filtering unit 83 uses the filter coefficient set to apply a weighted-average filtering operation on each pixel value in the filter window.