1. A method of removing a blocking artifact using quantization information in a filtering system, comprising: obtaining quantization information related to at least one of a first block and a second block, the second block being adjacent to the first block, comparing a value related to the quantization information with a constant value, determining whether the first block and the second block are intra-coded blocks, and performing a filtering process based on a result of the comparing step if both of the first block and the second block are determined to be intra-coded blocks, the filtering process including, calculating values of at least two pixels in the first block based on pixel values of the at least two pixels in the first block, and calculating values of at least two pixels in the second block based on pixel values of the at least two pixels in the second block.