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 that remains fixed over more than one execution of a filtering process for a plurality of frames to determine whether the quantization information is greater than or less than or equal to the predefined constant value; and performing the filtering process based on a result from the comparing step, the filtering process including, obtaining a filtered value of at least one pixel in the first block based on pixel values of at least two pixels in the first block, and obtaining a filtered value of at least one pixel in the second block based on pixel values of at least two pixels in the second block.