1. A pattern inspection apparatus comprising: an optical image acquiring unit which acquires optical image data of a target plate to be inspected, the target plate being formed as a pattern; a design image data generating unit which generates design image data based on a design pattern serving as a base of pattern formation of the target plate; and a comparing unit which inputs region image data generated based on information of a region pattern, representing a predetermined region and formed in the same format as that of information of the design pattern, and compares the optical image data with the design image data based on the region image data.