1. An image inspection apparatus for inspecting an image output on a recording medium by scanning the output image as a scanned image, the image inspection apparatus comprising: an inspection reference image generator to obtain data of an output-target image used by the image forming apparatus for an image fomiing operation, and to generate an inspection reference image using the data of the output-target image, the inspection reference image to be used for an image inspection of the scanned image; an image inspection unit to determine whether the scanned image includes a defect based on a comparison result obtained by comparing a difference between the inspection reference image and the scanned image with a given threshold; a threshold determiner to determine the given threshold based on a comparison result between the inspection reference image having a normal image condition and the scanned image of threshold setting image prepared by adding a plurality of artificial defects having different defect levels to the inspection reference image; and a defect range determiner to determine a range of defect level of a plurality of artificial defects having the different levels for the threshold setting image; wherein the threshold determiner controls generation of the inspection reference image having the normal image condition to be used for determining the given threshold, wherein the threshold determiner computes a difference between the inspection reference image and the scanned image of the threshold setting image for each of the plurality of artificial defects having the different defect levels, wherein, based on a difference computed for a defect selected from the plurality of artificial defects having different defect levels, the threshold determiner determines a threshold to be compared with the difference of the selected defect to determine whether the scanned image includes a defect; wherein the defect range determiner conducts a defect determination process for a range determination scanned image, obtained by scanning a range determination image displaying a plurality of defects having changed levels with a given interval at each of the upper limit and the lower limit settable for a threshold, wherein the defect range determiner determines a range of defect level of the plurality of artificial defects having the different defect levels for the threshold setting image based on a defect level of a defect determined as defect in the defect determination process for the range determination image at each of the upper limit and the lower limit settable for the threshold.