A decoding method performed by a decoding device, according to the present document, may comprise the steps of: acquiring, from a bitstream, at least one of a combined inter-picture merge and intra-picture prediction (CIIP) enable flag and a coding unit (CU) skip flag indicating whether a skip mode is applied to a current block; acquiring, from the bitstream, a regular merge flag on the basis that a condition based on the current block size, and at least one of a condition based on the CIIP enable flag and a condition based on the CU skip flag are satisfied; generating prediction samples of the current block on the basis of the regular merge flag; and generating restoration samples on the basis of the prediction samples.