A post-processing method for filtering decoded image signals determines positions of target pixels to be filtered and generates a selection signal indicating whether or not each of the target pixels belongs to a boundary region of a block of decoded image data. In response to the selection signal, each of the target pixel data is selectively filtered through the use of a filtering unit having two filters with different cutoff frequencies. An absolute difference value between filtered and non-filtered target pixel values is compared with a predetermined threshold value. The target pixel value is updated with the filtered target pixel value if the absolute difference value is smaller than the predetermined threshold value; otherwise, a compensated target pixel value is obtained by subtracting the predetermined threshold value from the non-filtered target pixel value or adding the non-filtered target pixel value and the predetermined threshold value, and, then the filtered target pixel value is updated with the compensated target pixel value.