A method for decoding an image in intra-prediction mode is discussed. The method includes reconstructing the intra-prediction mode of a current block; generating a reference pixel using an available reference pixel when there is an unavailable reference pixel for the current block; adaptively filtering the reference pixel of the current block based on the reconstructed intra-prediction mode and a size of the current block; and generating a prediction block of the current block using the intra-prediction mode of the current block and reference pixel corresponding to the intra-prediction mode Further, in the generating step of the reference pixel, the unavailable reference pixel is set as a pixel value of the available reference pixel, the available reference pixel being an available pixel on the nearest position in a predetermined direction from the unavailable reference pixel.