A method for inspecting pattern defects is provided to easily detect defects in a repetition pattern formed on a transparent substrate by irradiating light to a repetition pattern at a predetermined incidence angle by an illumination unit and by monitoring diffraction light generated by the light irradiation in a predetermined monitoring region on the repetition pattern by a monitoring unit. An object to be inspected is positioned on a transparent substrate(52) wherein a repetition pattern(51) in which unit patterns are periodically arranged is formed on the object. Light is irradiated to the repetition pattern at an incidence angle of theta I by an illumination unit(12). In a predetermined monitoring region on the repetition pattern, diffraction light formed by the irradiation of light is monitored by a monitoring unit(13) to detect whether a defect exits in the repetition pattern. At least part of the object to be inspected except the monitoring region is masked. The object to be inspected can be a photomask(50).
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