A video decoding method comprising the steps of: determining a reference sample line of a current block; determining candidate intra prediction modes; determining an intra prediction mode of the current block; determining whether a candidate intra prediction mode is the same as the intra prediction mode of the current block; deriving an intra prediction mode of the current block based on the determination; and performing intra prediction on the current block based on the reference sample line and the intra prediction mode, wherein at least one among the determined candidate intra prediction modes is derived by adding or subtracting an offset to or from a maximum index number value among an intra prediction mode of a top neighboring block of the current block and an intra prediction mode of a left neighboring block of the current block, wherein index number values are defined such that monotonously increasing index values are predetermined and mapped to a corresponding angular increase in predefined intra prediction directions, and wherein an index number value of 2 represents a bottom-left diagonal direction, and an index number value of 66 represents a top-right-diagonal direction, characterised by determining a difference between an index number of the intra prediction mode of the top neighboring block and an index number of the intra prediction mode of the left neighboring block, when said difference between the index number of the intra prediction mode of the top neighboring block and the index number of the intra prediction mode of the left neighboring block is 1 or 64, at least one among the candidate intra prediction modes is derived by adding or subtracting 2 to or from the maximum index number value; and when said difference between the index number of the intra prediction mode of the top neighboring block and the index number of the intra prediction mode of the left neighboring block is neither 1 nor 64, at least one among the candidate intra prediction modes is derived by adding or subtracting 1 to or from the maximum index number value. determining a reference sample line of a current block; determining candidate intra prediction modes; determining an intra prediction mode of the current block; determining whether a candidate intra prediction mode is the same as the intra prediction mode of the current block; deriving an intra prediction mode of the current block based on the determination; and performing intra prediction on the current block based on the reference sample line and the intra prediction mode, wherein at least one among the determined candidate intra prediction modes is derived by adding or subtracting an offset to or from a maximum index number value among an intra prediction mode of a top neighboring block of the current block and an intra prediction mode of a left neighboring block of the current block, wherein index number values are defined such that monotonously increasing index values are predetermined and mapped to a corresponding angular increase in predefined intra prediction directions, and wherein an index number value of 2 represents a bottom-left diagonal direction, and an index number value of 66 represents a top-right-diagonal direction, characterised by determining a difference between an index number of the intra prediction mode of the top neighboring block and an index number of the intra prediction mode of the left neighboring block, when said difference between the index number of the intra prediction mode of the top neighboring block and the index number of the intra prediction mode of the left neighboring block is 1 or 64, at least one among the candidate intra prediction modes is derived by adding or subtracting 2 to or from the maximum index number value; and when said difference between the index number of the intra prediction mode of the top neighboring block and the index number of the intra prediction mode of the left neighboring block is neither 1 nor 64, at least one among the candidate intra prediction modes is derived by adding or subtracting 1 to or from the maximum index number value.