A decoding method carried out by a decoding device, according to the present document, comprises the steps of: acquiring, from a bitstream, information associated with a prediction mode of a current block; deriving a prediction mode of the current block on the basis of the information associated with the prediction mode; deriving prediction samples of the current block on the basis of the prediction mode; and generating reconstructed samples on the basis of the prediction samples, wherein the bitstream comprises a sequence parameter set, the sequence parameter set comprising a combined inter-picture merge and intra-picture prediction (CIIP) enable flag, and the step for deriving may comprise a step for parsing a regular merge flag from the bitstream on the basis of the satisfying of a condition based on the CIIP enable flag and a condition based on the size of the current block.