1. A decoding apparatus comprising: a decoder configured to obtain image data from an encoded image data by orthogonal transformation decoding; a de-ringing filter configured to filter the image data, wherein the de-ringing filter comprising: a subtracter configured to generate an absolute value of difference between a value of a filter object pixel and a value of at least one pixel selected from pixels surrounding the filter object pixel; a comparator configured to compare the absolute value with a threshold; a selector configured to output the value of the at least one pixel if the absolute value is less than the threshold, and to output the value of the filter object pixel if the absolute value is not less than the threshold; and a convolution operator configured to convolute a filter coefficient with the value output from the selector, and to output a convolution result as a filtered value of the filter object pixel.