An image decoding method performed by a decoding device according to the present document comprises the steps of: deriving a temporary reference motion vector regarding a current block on the basis of a temporal periphery block included in a reference picture of a current picture including the current block; deriving a binary code indicating a reference motion vector by applying a shifting operation to a binary code indicating the temporary reference motion vector; storing information regarding the binary code indicating the reference motion vector; deriving a motion vector regarding the current block on the basis of the reference motion vector; deriving a predicted block regarding the current block on the basis of the motion vector regarding the current block; and generating a restored picture regarding the current picture on the basis of the predicted block regarding the current block.