To sufficiently remove even block distortion which is small in a difference value of a signal at a border of a block without spoiling a high--frequency component of a source image.

When a pixel signal included in pixel blocks forming one image is supplied, a detection part 1 detects a difference in pixel signal level at the periphery of the border of pixel blocks. This difference is compared by a threshold value decision part 2 with a reference value. According to the result of the comparison, an adder 5 adds a specific addition value to the pixel level at the periphery of the said block border. When the specific addition value is added, a random number is generated and based on this random number, a pattern containing the said specific addition value is selected out of addition value patterns. Consequently, the regularity of the difference value at the border of the pixel blocks is disordered and the block distortion generated between pixel blocks can be removed.