A decoding method performed by a decoding device, according to the present document, may comprise the steps of: acquiring a sequence parameter set from a bitstream; acquiring a combined inter-picture merge and intra-picture prediction (CIIP) enable flag from the sequence parameter set; determining, on the basis of the CIIP enable flag and the size of a current block, whether a regular merge flag is included in the bitstream; parsing the regular merge flag in the bitstream on the basis that a condition based on the CIIP enable flag and a condition based on the size of the current block are satisfied; generating prediction samples of the current block on the basis of the regular merge flag; and generating a restoration picture on the basis of the prediction samples.