1. An inspection apparatus to inspect a read image that is read from a printed image output from an image forming apparatus, the inspection apparatus comprising: a processor to: obtain image data of the printed image, which is used by the image forming apparatus to form the printed image; generate a master image based on the image data of the printed image, the master image being added with a plurality of units of pattern; store pattern location information indicating a specific location in the master image to which a specific unit of pattern is added, for each one of the plurality of units of pattern; extract one or more corners of the master image; store corner location information indicating a specific location in the master image from which a specific corner is extracted, for each one of the one or more corners of the master image; select at least one of the plurality of units of pattern and the one or more corners of the master image as a reference point to be used for detecting a positional shift between the read image and the master image, using the pattern location information and the corner location information; extract a reference point in the read image, which corresponds to the selected reference point in the master image; correct pixel positions of one of the read image or the master image based on the positional shift between the reference point in the read image and the reference point in the master image to generate a corrected image; and inspect the read image based on difference between the corrected image and the other one of the read image or the master image that is not corrected to generate an inspection result indicating whether the printed image sufficiently reproduces the image data of the printed image.