1. An image processing apparatus, comprising: an averaging processing section for periodically performing averaging processing on image data of an averaging target, the averaging processing comprising calculating an average value of pixel values of pixels which correspond to a position of a one-dimensional or two-dimensional averaging pattern designating averaging target pixels and replacing each of the pixel values of the pixels which correspond to the position of the one-dimensional or two-dimensional averaging pattern with the average value; and a screen processing section for, using a screen pattern, periodically performing screen processing on the image data on which the averaging processing has been performed, a periodicity of the screen processing being the same as that of the averaging processing; wherein when an image discriminating signal showing a property of each pixel of the image data is input, the averaging processing section excludes pixels belonging to a specific region among target pixels in the one-dimensional or two-dimensional averaging pattern from the target pixels of the averaging processing and calculates the average value of pixel values of pixels not belonging to the specific region in the one-dimensional or two-dimensional averaging pattern, based on the image discriminating signal.