1. An image processing system for performing filtering on a boundary pixel at the boundary between neighboring blocks, said block being a unit of discrete cosine transform in image compression, said image processing system, characterized by comprising: determination means for referencing 2N pixels, including said boundary pixel, that consist of N pixels on each side of said boundary and run orthogonally to said boundary to determine whether correction should be performed, where N is an integer between 3 and 5 inclusive; the same applies to the following description; and correction means for performing correction by using said boundary pixel and each set of M pixels on each side of said boundary pixel if said determination means determines that correction should be performed, where M is an integer smaller than N; the same applies to the following description.