A video decoding method comprising: generating a prediction block by performing intra prediction on a current block for a DC mode, and by performing filtering on a filtering target pixel in the prediction block; and generating a reconstructed block based on the prediction block and a reconstructed residual block corresponding to the current block, wherein the filtering target pixel is one of a plurality of filtering target pixels comprising one or more pixels on a left vertical prediction pixel line that is one vertical pixel line positioned at a leftmost side of the prediction block and one or more pixels on an upper horizontal prediction pixel line that is one horizontal pixel line positioned at an uppermost side of the prediction block, wherein the filtering is performed based on a predetermined fixed filter shape, a predetermined fixed filter tap and a plurality of predetermined fixed filter coefficients for a plurality of block sizes of the current block, wherein in a case that the filtering target pixel is a left upper prediction pixel positioned at a leftmost upper side of the prediction block, the filtering is performed on the filtering target pixel by applying a 3-tap filter based on the filtering target pixel, an upper reference pixel adjacent to an upper portion of the filtering target pixel, and a left reference pixel adjacent to a left portion of the filtering target pixel, each of the upper reference pixel and the left reference pixel is a reconstructed reference pixel adjacent to the current block, and in the 3-tap filter, a filter coefficient allocated to a filter tap corresponding to the filtering target pixel is 2/4, a filter coefficient allocated to a filter tap corresponding to the upper reference pixel is 1/4, and a filter coefficient allocated to a filter tap corresponding to the left reference pixel is 1/4 for the plurality of block sizes of the current block, and wherein the filtering is performed in a case that the current block has a size of 4 x 4, 8 x 8 or 16 x 16. generating a prediction block by performing intra prediction on a current block for a DC mode, and by performing filtering on a filtering target pixel in the prediction block; and generating a reconstructed block based on the prediction block and a reconstructed residual block corresponding to the current block, wherein the filtering target pixel is one of a plurality of filtering target pixels comprising one or more pixels on a left vertical prediction pixel line that is one vertical pixel line positioned at a leftmost side of the prediction block and one or more pixels on an upper horizontal prediction pixel line that is one horizontal pixel line positioned at an uppermost side of the prediction block, wherein the filtering is performed based on a predetermined fixed filter shape, a predetermined fixed filter tap and a plurality of predetermined fixed filter coefficients for a plurality of block sizes of the current block, wherein in a case that the filtering target pixel is a left upper prediction pixel positioned at a leftmost upper side of the prediction block, the filtering is performed on the filtering target pixel by applying a 3-tap filter based on the filtering target pixel, an upper reference pixel adjacent to an upper portion of the filtering target pixel, and a left reference pixel adjacent to a left portion of the filtering target pixel, each of the upper reference pixel and the left reference pixel is a reconstructed reference pixel adjacent to the current block, and in the 3-tap filter, a filter coefficient allocated to a filter tap corresponding to the filtering target pixel is 2/4, a filter coefficient allocated to a filter tap corresponding to the upper reference pixel is 1/4, and a filter coefficient allocated to a filter tap corresponding to the left reference pixel is 1/4 for the plurality of block sizes of the current block, and wherein the filtering is performed in a case that the current block has a size of 4 x 4, 8 x 8 or 16 x 16.