In a halftone image formed on a printing plate for letterpress printing, a minimum halftone dot has a square main cluster consisting of 9 pixels and four sub-clusters located around the main cluster radially, each of which is one pixel. Each sub-cluster contacts with only a vertex of the main cluster. Convex portions corresponding to the minimum halftone dots in the halftone image are formed on the printing plate. A convex portion corresponding to the main cluster is supported by convex portions corresponding to the sub-clusters to increase the strength of relief corresponding to the minimum halftone dot. Reliefs corresponding to the minimum halftone dots are normally formed regardless of a dot area rate of highlight area to thereby improve reproduction in printing of the halftone image.