Two sets of image data, a first image and a second image, are prepared as image data. If image controlling information is received, an image setting controlling section determines, on the basis of this information, area dividing information which allocates one of the first image and the second image to each of the areas into which an image plane is divided in the same coordinate system. A decoding section decodes the two sets of image data for the respective first and second image that were loaded into a main memory, determines which image data to adopt for each pixel based on the area dividing information, and writes the adopted image data to a buffer memory. The decoding process and writing process are carried out in tile image units of a predetermined size using a working memory.