A method of inspecting a defect present at a surface portion of a photomask blank comprising at least one thin film formed on a substrate, by use of an inspecting optical system, the method comprising: a step of bringing the defect and an objective lens of the inspecting optical system close to each other, setting a distance between the defect and the objective lens to a focus distance, and, with the focus distance thus set, applying inspection light to the defect through the objective lens; a step of collecting reflected light from a region irradiated with the inspection light, through the objective lens, as a first magnified image of the region; a first determination step of identifying a light intensity variation portion of the first magnified image and determining a rugged shape of the defect on the basis of a variation in light intensity of the light intensity variation portion of the first magnified image (S205, S206); a step of setting the distance between the defect and the objective lens of the inspecting optical system to a defocus distance deviated from the focus distance, and, with the defocus distance thus set, applying the inspection light to the defect through the objective lens; a step of collecting reflected light from a region irradiated with the inspection light, through the objective lens, as a second magnified image of the region; and a second determination step of identifying a light intensity variation portion of the second magnified image and re-determining the rugged shape of the defect on the basis of a variation in light intensity of the light intensity variation portion of the second magnified image (S209,210); Characterized in that when defect shape is determined to be a recessed shape in the first determination step, the steps to are carried out to re-determine the rugged shape of the defect. a step of bringing the defect and an objective lens of the inspecting optical system close to each other, setting a distance between the defect and the objective lens to a focus distance, and, with the focus distance thus set, applying inspection light to the defect through the objective lens; a step of collecting reflected light from a region irradiated with the inspection light, through the objective lens, as a first magnified image of the region; a first determination step of identifying a light intensity variation portion of the first magnified image and determining a rugged shape of the defect on the basis of a variation in light intensity of the light intensity variation portion of the first magnified image (S205, S206); a step of setting the distance between the defect and the objective lens of the inspecting optical system to a defocus distance deviated from the focus distance, and, with the defocus distance thus set, applying the inspection light to the defect through the objective lens; a step of collecting reflected light from a region irradiated with the inspection light, through the objective lens, as a second magnified image of the region; and a second determination step of identifying a light intensity variation portion of the second magnified image and re-determining the rugged shape of the defect on the basis of a variation in light intensity of the light intensity variation portion of the second magnified image (S209,210); Characterized in that when defect shape is determined to be a recessed shape in the first determination step, the steps to are carried out to re-determine the rugged shape of the defect.