A method and apparatus for deriving an intra-prediction mode are disclosed. At least one embodiment of the present disclosure provides a method of deriving an intra-prediction mode, including (i) obtaining, based on reference line information indicating any one of a plurality of reference lines, most probable mode (MPM) information indicating whether an intra-prediction mode of a current block is included in an MPM list lacking a planar mode, (ii) determining, based on the reference line information and the MPM information, planar information indicating whether the intra-prediction mode of the current block is the planar mode, and (iii) deriving the intra-prediction mode of the current block by using the planar information.