A halftone dot threshold data rewriting unit reads halftone dot threshold data corresponding to the output resolution of a printed image generating apparatus which has been set by an output resolution setting unit, from a halftone dot threshold data storage unit, rewrites the read halftone dot threshold data into halftone dot threshold data corresponding to the output resolution of a proof image generating apparatus, and supplies the rewritten halftone dot threshold data to a binary image generator. The binary image generator compares continuous tone image data with the halftone dot threshold data, generates binary image data, and supplies the generated binary image data to an exposure recorder, which generates a proof image corresponding to the output resolution of the printed image generating apparatus.