To reliably decide irregularity in an area gradation image at low cost.

The area gradation image process method includes steps of: acquiring color characteristics information (step S6) to acquire color density data relating to the density of pixels colors constituting a proof image on the basis of the dot image data of a proof image with gradation expressed by area gradation and comprising a plurality of colors; acquiring period characteristics (step S7) to acquire the Wiener spectral information of the proof image on the basis of the acquired color density data; and deciding irregularity (step S8) to decide irregularity in the proof image on the basis of the acquired Wiener spectral information.