Image decoding method and device in image coding system

An image decoding method performed by a decoding device according to the present invention comprises: a step of deriving L0 and L1; a step of receiving, through a bitstream, a syntax element indicating L0 motion information and L1 motion information of a current block; a step of deriving the L0 motion information and the L1 motion information of the current block on the basis of the syntax element; and a step of generating a prediction sample of the current block on the basis of the L0 motion information and the L1 motion information, wherein the L0 motion information includes at least one of a motion vector predictor L0 and an index of a first reference picture of the current block, the motion vector predictor L0 and the first reference picture being included in the L0, and the L1 motion information includes at least one of a motion vector predictor L1 and an index of a second reference picture of the current block, the motion vector predictor L1 and the second reference picture being included in the L1.