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 the quantization information with a predefined constant value; and performing a filtering process based on a result from the comparing step, the filtering process including, calculating a value of at least one pixel in the first block based on pixel values of at least two pixels in the first block, and calculating a value of at least one pixel in the second block based on pixel values of at least two pixels in the second block.