1. A pattern image correcting apparatus for correcting an inspection reference pattern image in a pattern inspection, comprising: a first pattern synthesizing unit for synthesizing an assist pattern image and a pattern image to be inspected, thereby generating a pattern image to be inspected with an assist pattern; an assist pattern shift processing unit for shifting the assist pattern image on the basis of an amount of a relative positional deviation between the inspection reference pattern image and the pattern image to be inspected; a second pattern synthesizing unit for synthesizing the shifted assist pattern image and the inspection reference pattern image, thereby generating an inspection reference pattern image with an assist pattern; a model generating unit for generating a position shift model by using the pattern image to be inspected with the assist pattern and the inspection reference pattern image with the assist pattern; and a correction pattern image computing unit for correcting the inspection reference pattern image on the basis of the position shift model.