The present invention enables the correction of photomask defects without requiring the time and the number of steps, thereby improving production efficiency or quality in manufacturing a photomask. A photomask substrate correction method comprises: a process of preparing a photomask substrate (10) having an optical film (200) for forming a transfer pattern on one main surface of a transparent substrate (100); and a correction process of forming a correction film for cut defects formed on the optical film (200). The correction process comprises: supplying raw material gas to the vicinity of the position of the cut defect on a first main surface on which the optical film (200) of the photomask substrate (10) is formed and concurrently emitting a laser beam from a second main surface side of the photomask substrate (10); and causing a reaction of the raw material gas by the laser beam that has transmitted the cut defect to result in deposition of the correction film at the position of the cut defect on the first main surface.
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