A calculation is made as to first and second differences in level represented by an input video signal at first and second inter-block boundaries adjoining opposite edges of every rectangular block respectively. A corrective value at every pixel in the rectangular block is calculated from a position of the pixel and the first and second differences according to a predetermined equation. The corrective value is added to a level represented by the input video signal to get an output video signal. The predetermined equation is designed so that levels represented by the output video signal at respective pixels in the rectangular block will be on a line given by a predetermined function of pixel position and connecting a first point in a range corresponding to the first difference and a second point in a range corresponding to the second difference.