A decoding method performed by a decoding device according to the present document comprises the steps of: determining a prediction mode of a current block on the basis of information about a prediction mode obtained from a bitstream; constructing a merge candidate list on the basis of the prediction mode; deriving motion information of the current block on the basis of the merge candidate list; and generating prediction samples of the current block on the basis of the motion information, wherein the bitstream includes information about a combined inter-picture merge and intra-picture prediction (CIIP) available flag indicating whether the CLIP is available, and the determination step may include a step of acquiring a regular merge flag from the bitstream on the basis of the CIIP available flag.