To eliminate the seeming unnaturalness of an image having information on transparency when the image is displayed in a state where the image is superimposed upon another image by enlarging/reducing the image.

When an interpolation picture element is obtained from a plurality of original picture elements having different transparency values at the time of enlarging the image composed of picture elements having information on transparency, an original picture element is selected from the original picture elements according to a prescribed rule and the transparency value of the selected picture element is set to the interpolation picture element instead of obtaining the transparency value through picture element interpolation. Consequently, no area having an intermediate transparency value is generated in the area corresponding to the boundary between unenlarged/unreduced picture elements having different transparency values in an enlarged/reduced image.