1. An encoding method comprising: performing a RMD (Rough Mode Decision) process comprising calculating a first cost of each intra-prediction mode with respect to a block to be encoded using a Hadamard transform and calculating a second cost of each intra-prediction mode with respect to the block using a sum of absolute values of differences between original pixels and reference pixels; selecting first candidate modes of the intra-prediction modes according to the first cost; selecting second candidate modes of the intra-prediction modes according to the second cost; performing a RDO (Rate Distortion Optimization) process comprising calculating a DCT (Discrete Cosine Transform)-based rate-distortion cost according to the first candidate modes; and calculating a TSM (Transform Skip Mode)-based rate-distortion cost according to the second candidate modes; and comparing the rate-distortion costs of the first and second candidate modes and deciding a mode and an encoding method to be used for intra-prediction of the block, wherein the RMD process is divided into two steps of calculating using the Hadamard transform and calculating using the sum of absolute values of differences between original pixels and reference pixels.