1. A calibration method of performing contrast threshold calibration in extracting a pattern edge from an image of a pattern formed on a processing target substrate, comprising: simulating formation of the pattern to detect a portion predicted to have shorted or opened in the pattern; calculating a contrast of an image of the pattern including the portion predicted to have shorted or opened; and determining a threshold from the contrast, the threshold avoiding extraction of a pattern edge in the portion predicted to have shorted or opened.