In order to combine N (N≧3) pieces of image data including not only color data but also distance data and opacity data, when images that are to be combined are input, distance data of each pixel in each image data is sorted in parallel for each pixel data at identical coordinates in a plurality of images so as to determine the distance relationship as the order relationship concerning the input image data. On the other hand, color data and opacity data of each input image that are to be combined are stored in a pixel-by-pixel manner. The color data and opacity data stored in the storing step are read and rearranged in parallel by using the order relationship concerning the input image data obtained in the sorting step so that the color data and opacity data are arranged in the order relationship corresponding to the input images. The color data and opacity data rearranged in the rearranging step are combined, in an alpha blending technique, in parallel.