An original image to be edited is displayed using hierarchical data. As a user draws a figure in a region of the image as an edit action, an image data updating unit generates a layer having a hierarchical structure composed of the rendered region only. More specifically, the image of the region to be edited is used as the lowermost hierarchical level, and upper hierarchical levels than this lowermost level are generated by reducing the lowermost level, as appropriate, so as to produce hierarchical data. As, during image display, it is checked that the updated region is contained in a frame to be displayed anew, the image of the layer is displayed by superposing the frame on the original hierarchical data.