This invention makes it possible to obtain an enlarged image while suppressing noise and maintaining the sharpness of an original image only by setting a simple enlargement ratio without no special knowledge. Original image data (Ia) is enlarged in accordance with a set enlargement ratio (E) to generate an enlarged image (IA). The enlarged image (IA) is smoothed by using a smoothing filter with a size depending on the enlargement ratio (E) to generate smoothed image data (IB). Difference image data (IC) is generated by calculating the difference between the enlarged image data (IA) and the smoothed image data (IB). The generated difference image data is multiplied by an emphasis coefficient. The product is added to the enlarged image data, thereby obtaining image data (IS) that has undergone enlargement/unsharp masking.