An image decoding method performed by the decoding apparatus according to this document comprising deriving a temporary reference motion vector for a current block based on a reference candidate block of the current block, deriving a reference motion vector by compressing the temporary reference motion vector based on a shifting operation, deriving a motion vector for the current block based on the reference motion vector, deriving prediction samples for the current block based on the motion vector for the current block, and generating reconstructed samples for the current block based on the prediction samples for the current block, wherein a total number of bits of the binary code representing the reference motion vector is less than a total number of bits of the binary code representing the temporary reference motion vector.