An intra prediction apparatus, the apparatus comprising: a prediction mode decoder configured to restore an intra prediction mode on the basis of intra prediction information; a reference pixel generator configured to generate reference pixels corresponding to unavailable reference pixels and to adaptively filter the reference pixels according to the intra prediction mode; a prediction block generator configured to generate a prediction block using reference pixels determined by the intra prediction mode; and a prediction block filter configured to adaptively filter a part of the prediction block using the intra prediction mode, wherein the reference pixel generator adaptively filters the reference pixels according to the size of the prediction block for intra prediction modes existing between a horizontal mode and an intra prediction mode having a direction of 45° with respect to the horizontal mode, wherein, when the reference pixels are filtered for a second directional intra prediction mode, the reference pixels are also filtered for a first directional intra prediction mode that is closer to the intra prediction mode having the direction of 45° with respect to the horizontal mode than the second directional intra prediction mode is, wherein the second and the first directional intra prediction modes exist between the horizontal mode and the intra prediction mode having the direction of 45° with respect to the horizontal mode, wherein the filter is applied to the intra prediction mode having the direction of 45° with respect to the horizontal mode and is not applied to the horizontal mode and a vertical mode, wherein the number of directional modes in which the filter is applied increases as the size of the prediction block increases, wherein the reference pixel generator does not filter the reference pixels of the prediction block which is smaller than a predetermined size. a prediction mode decoder configured to restore an intra prediction mode on the basis of intra prediction information; a reference pixel generator configured to generate reference pixels corresponding to unavailable reference pixels and to adaptively filter the reference pixels according to the intra prediction mode; a prediction block generator configured to generate a prediction block using reference pixels determined by the intra prediction mode; and a prediction block filter configured to adaptively filter a part of the prediction block using the intra prediction mode, wherein the reference pixel generator adaptively filters the reference pixels according to the size of the prediction block for intra prediction modes existing between a horizontal mode and an intra prediction mode having a direction of 45° with respect to the horizontal mode, wherein, when the reference pixels are filtered for a second directional intra prediction mode, the reference pixels are also filtered for a first directional intra prediction mode that is closer to the intra prediction mode having the direction of 45° with respect to the horizontal mode than the second directional intra prediction mode is, wherein the second and the first directional intra prediction modes exist between the horizontal mode and the intra prediction mode having the direction of 45° with respect to the horizontal mode, wherein the filter is applied to the intra prediction mode having the direction of 45° with respect to the horizontal mode and is not applied to the horizontal mode and a vertical mode, wherein the number of directional modes in which the filter is applied increases as the size of the prediction block increases, wherein the reference pixel generator does not filter the reference pixels of the prediction block which is smaller than a predetermined size.