An image decoding method, which is performed by a decoding device according to the present invention, comprises the steps of: acquiring the information on an intra-prediction mode of a current block from a received bit stream; deriving neighboring reference samples of the current block; deriving a first prediction value for a current sample of the current block on the basis of the intra-prediction mode and the neighboring reference samples; deriving a second prediction value for the current sample on the basis of adjacent samples of the current sample; and deriving a prediction value for the current sample on the basis of the first prediction value and the second prediction value. According to the present invention, the intra-prediction value can be derived on the basis of the adjacent samples of the current sample within the present block, and the intra-prediction accuracy can be improved therethrough.