PURPOSE: To eliminate the influence of temperature variation and a secular change, and to correct even gentle variation due to shading, by digitizing a video signal through A/D conversion, and performing correction through digital arithmetic.

CONSTITUTION: A sheet 1 is scanned to output a video signal from an image sensor 4, and the video signal is digitized by an A/D converter 10 to write the digitized image signal in a memory 12 through a memory writing control circuit 11. This operation is repeated on every scan on the sheet 1 to hold a maximum white value in the sheet 1 in the memory 12, and when the reading of the sheet 1 ends, clearing operation is performed by a memory clear signal 15. On the basis of the contents of this memory 12, the output of a constant generator 13 is selected for multiplication, and this output is multiplied by the original video signal at a multiplier 14, whose output is made uniform when the video signal has a white level and coded by a binary coding circuit 9, eliminating the influence of temperature variation and a secular change.