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 includes, 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 first threshold; a selector configured to output the value of the at least one pixel if the absolute value is less than the first threshold, and to output the value of the filter object pixel if the absolute value is not less than the first 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; a vertical de-block filter configured to calculate an absolute difference value between each unit of two neighboring pixels along a vertical pixel line crossing a horizontal block boundary of the image data, to count a number of units of which the absolute difference value is less than a second threshold, to decide whether the number of the units is less than a third threshold, and to partially correct values of pixels on the vertical pixel line based on the decision result; and a horizontal de-block filter configured to calculate an absolute difference value between each unit of two neighboring pixels along a horizontal pixel line crossing a vertical block boundary of the image data, to count number of units of which the absolute difference value is less than the second threshold, to decide whether the number of the units is less than the third threshold, and to partially correct values of pixels on the horizontal pixel line based on the decision result.