When an original image is to be printed as a poster composed of an array of 4 × 4 output images, an image is generated by enlarging the original image fourfold in the vertical and horizontal directions and dividing the enlarged image into 16 parts. These 16 partial images are output successively starting from the upper left or lower right of the 4 × 4 array, depending upon the designation. If there is a blank page, the blank page is output as is even if the printer possesses a blank-paper economizing function. Thus, when a poster printing function is used, the user designates the manner in which the enlarged image is divided up and the order in which the divided images are printed. Even if the printer is equipped with the blank-paper economizing function, normal printed results can be obtained merely by joining the output sheets together in the order that conforms to the manner in which the enlarged image was divided.