The inspection system arbitrarily selects from among a plurality of optical conditions to change a distribution of reflected or diffracted light component from an object being inspected. The inspection system has a one- or two-dimensional optoelectric conversion image sensor, optically acquires an image of the object by scanning a stage on which the object is mounted or scanning the image sensor, and processes the image to check for defects in the object. Under each optical condition (illumination optical system, detection optical system, scan direction, etc.) the object being inspected is imaged and, based on the brightness distribution and contrast in the detection field of the image sensor, image sensor output correction data is generated to correct the output of the image sensor.