A reference image is generated by converting a vector image into a raster image, a temporarily-compressed image is generated by compressing the raster image according to a compression ratio, a comparison image of the same size as the reference image is generated by subjecting the temporarily-compressed image to interpolation enlargement processing, the above processing is repeated while varying the compression ratio if the error ratio between the reference image and the comparison image is greater than a benchmark error ratio, the above processing is repeated while varying the benchmark error ratio if the image volume of the temporarily-compressed image is greater than a predetermined memory capacity when the error ratio is at or under the benchmark error ratio, and the temporarily-compressed image is stored in memory as a compressed image of a vector image if the image volume of the temporarily-compressed image is at or under the memory capacity.