obtaining an intra prediction mode related to a current block from a bitstream;

obtaining prediction samples of the current block by performing intra prediction on the current block based on the intra prediction mode;

scanning transform coefficients of the current block,

obtaining dequantized transform coefficients of the current block by performing a dequantization on the transform coefficients;

determining a transform type of the current block; and

obtaining residual samples of the current block by performing an inverse-transform on the dequantized transform coefficients based on the determined transform type, the inverse-transform being performed in a horizontal direction and a vertical direction,

wherein the transform type is determined to be a DCT (Discrete Cosine Transform) or a DST (Discrete Sine Transform),

wherein when a size of the current block is 4×4 the DST is applied both in the horizontal direction and the vertical direction for all intra prediction direction mode and a planar mode of the current block.