1. An image processing apparatus, comprising: a periodicity pattern analyzing section to analyze image data representing a processing objective image, so as to acquire periodicity information indicating a periodicity of a halftone dot pattern included in the processing objective image; a smoothing area determining section to determine a smoothing area, constituted by either a pair of rectangular areas contacting with each other or a single rectangular area, based on the periodicity information acquired by the periodicity pattern analyzing section; a spatial filter creating section to create a spatial filter within a size of which the smoothing area, determined by the smoothing area determining section, is includible, and in which a weighted coefficient of each of pixels, included in the spatial filter, is established corresponding to an overlapping area of a concerned pixel overlapping with the smoothing area included in the spatial filter; and a smoothing calculation section that applies a convolution arithmetic calculation processing to the image data representing the processing objective image with the spatial filter, created by the spatial filter creating section.