An image encoding method may include acquiring image information including inter prediction mode information and residual information through a bitstream, generating residual samples based on the residual information, generating prediction samples of a current block by applying a prediction mode determined based on the inter prediction mode information, and generating reconstructed samples based on the prediction samples and the residual samples. Based on a CIIP mode being not enabled, a partitioning prediction mode in which prediction is performed by dividing the current block into two partitions being not enabled, a value of the first flag for a subblock merge mode being equal to 0 and a value of the third flag for a MMVD being equal to 0, the inter prediction mode information includes merge index information indicating one of merge candidates included in a merge candidate list generated for the regular merge mode.