1. A pattern inspection method comprising: scanning a substrate on which patterns are formed with a charged beam, detecting a charged particle generated from the surface of the substrate, and then acquiring an image of the patterns; comparing the image of the patterns with CAD data for the patterns to inspect the patterns; measuring the dimensions of an arbitrary pattern using the image; calculating a statistic of a dimensional value of the arbitrary pattern obtained by the measurement; judging the necessity of a correction on the basis of the calculated statistic; and performing correction processing when the correction is judged to be necessary.