To optimize the correction shape for OPC mask defects and to ensure exact execution of decision on quality by executing light intensity distribution simulation under prescribed conditions using image data of a pattern region which includes the defect portion of an OPC pattern.

When the pattern region inclusive of the defect portion enters the observation visual field of a device and is subjected to image observation, the computer-processable image data is captured by an image data capture means 23. The captured image data are converted into pattern data for light intensity simulation after the observation image, and the pattern data fetched from an OPC pattern database 32 are combined in an image data compositing and converting means 24. Light intensity simulation 26 is executed by using the converted pattern data. Furthermore, the simulation results are analyzed and evaluated, and optimum correction shape is determined, by which the correction processing using a defect correcting means 22 is executed.