1. An inspection system comprising: a light source for illuminating an object to be inspected; a stage mounting the object; an optical system to change a distribution of reflected or diffracted light component from the object; and a one- or two-dimensional optoelectric conversion image sensor to image the object through the optical system, the sensor including sensor pixels wherein the stage and the image sensor are scanned relative to each other to obtain an image of the object and, based on the image, the object is inspected for defects, said inspection system further comprising a sensor output correction unit to correct a contrast in a viewing field of the image sensor, wherein the object to be inspected has a repeating pattern having a recurring pattern interval, wherein said sensor output correction unit comprises: a contrast calculation unit to calculate, based on output values output from the image sensor obtained by changing an illuminating light quantity of the light source illuminating the object to be inspected in a stepwise manner, an output average of the sensor pixels for each recurring pattern interval of the object at each illuminating light quantity, and to calculate, based on the output average of the sensor pixels at said each illuminating light quantity, contrasts near at least ends or center of the viewing field of the image sensor; an object polynomial coefficient calculating unit to calculate, for all of the sensor pixels, an output average for each recurring pattern interval, from the output average in each recurring pattern interval of those sensor pixels corresponding in position to at least the ends or center of the viewing field of the image sensor, and to calculate a coefficient of a curve approximation polynomial, the curve approximation polynomial having, for each sensor pixel, as its input the calculated output average for each recurring pattern interval and as its output an output average for each recurring pattern interval of a sensor pixel whose contrast is highest; and a correction coefficient storage unit to store the calculated coefficient.