A white-plate graphic erosion process is performed on print data subjected to a RIP process in the following procedure. First, an erosion candidate region which is a candidate for a region in which a white-plate graphic is eroded is determined. Then, a region to be applied with an ink of a color other than white is determined. Then, an application target region is determined. Then, a region other than a region obtained by an erosion process out of the application target region is determined to be an erosion allowed region. Then, a region included in the colored region and the erosion allowed region out of the erosion candidate region is determined to be an erosion target region. Finally, the values of data of pixels included in the erosion target region among white-plate data are rewritten such that the amount of white ink applied to the pixels is reduced.