A correction method according to an embodiment includes illuminating an object to be inspected by using critical illumination by illumination light L11 generated by a light source 11, concentrating light from the object to be inspected illuminated by the illumination light L11 and acquiring image data of the object to be inspected by detecting the concentrated light by a first detector 23, concentrating part of the illumination light L11, and acquiring image data of a brightness distribution of the illumination light L11 by detecting the concentrated illumination light L11 by a second detector 33, and correcting the image data of the object to be inspected based on the image data of the brightness distribution.