PURPOSE: To obtain high sensitivity and high dynamic range characteristic by adding an offset level to an output of an image sensor corresponding to high sensitivity or low sensitivity relatively.

CONSTITUTION: A chart having gradation whose contrast is continuously changed is used and a position corresponding to a border is searched and marked at an incident light quantity when an output of a CCD 1B is '00001' and suppose that an output of a CCD 1A is '1111100', then a value '0000100' adding '0000001' to '0000011' resulting from subtracting '1111100' from '1111111' is calculated as an offset and after the offset '0000100' is given to a D/A converter, from which an analog voltage is fed to an offset adder section, in which the voltage is added to output data of the CCD 1A, then two characteristic curves are jointed continuously to correct a gradation step difference.