To give linearity to data output to suppress the occurrence of a data difference in the center part and the end part of a visual field, and to make confirmable abnormal output data.

The number of bits in an A/D converter 1 is made larger than the number of output bits, and an offset memory 12 which calculates offset data is provided. Offset data calculated by lower bits, which are unnecessary originally, and each shading data is added to original output data by an adder 13.