A decoding apparatus having a de-ringing filter to filter image data decoded from encoded image data by orthogonal transformation encoding. In the de-ringing filter, a subtracter generates 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 on the image data. A comparator compares the absolute value with a threshold. A selector outputs the value of the at least one pixel if the absolute value is less than the threshold, and outputs the value of the filter object pixel if the absolute value is not less than the threshold. A convolution operator convolutes a filter coefficient with the value output from the selector, and outputs a convolution result as a filtered value of the filter object pixel.