A mask inspection apparatus including, a driving unit configured to drive a stage holding an inspection target mask, in which a pattern is formed, or a calibration mask, a light irradiation device configured to irradiate light on the inspection target mask or the calibration mask, an image sensor configured to detect a light quantity signal of transmitted light or reflected light of the inspection target mask or the calibration mask at a plurality of pixels. A sensor amplifier configured to amplify an output of the image sensor with respect to each pixel, generates an optical image, and normalizes a gain and an offset of signal amplitude, wherein at a first setting the sensor amplifier sets the gain and the offset using the calibration mask, and at a second setting the sensor amplifier sets the gain and offset of the inspection target mask based on the first setting.