1. An image data filtering apparatus for reducing a blocking effect, the apparatus comprising: a checker which checks whether a transformed coefficient having a non-zero value, among transformed coefficients obtained from a data block, exists in the data block; a filter information generator which generates filtering information indicating whether to perform deblocking filtering for the data block according to whether the transformed coefficient having the non-zero value exists in the data block; and a filter which filters a boundary pixel of the data block passed through inverse quantization and inverse transform according to the generated filtering information.