An image decoding method performed by a decoding apparatus, comprising: deriving an intra prediction type for a current block as Multi Reference Line intra prediction based on prediction related information, wherein the MRL is an intra prediction type based on an intra prediction reference line not adjacent to the current block; constructing an intra prediction mode candidate list including a DC intra prediction mode based on intra prediction modes of neighboring blocks of the current block; deriving an intra prediction mode of the current block based on the intra prediction mode candidate list; deriving a prediction sample of the current block based the intra prediction mode; deriving a residual sample of the current block based on residual information; and generating a reconstructed sample based on the prediction sample and the residual sample, characterized in that, based on the intra prediction mode of the current block being derived as the DC intra prediction mode, the prediction sample is derived as a DC value, wherein the DC value is derived based on a width of the current block and a height of the current block, (i) wherein based on the width of the current block being equal to the height of the current block, the DC value is derived based on an equation as follow: dcVal = ∑ x ′ = 0 nTbW − 1 p x ′ − 1 − refIdx + ∑ y ′ = 0 nTbH − 1 p − 1 − refIdx y ′ + nTbW ≫ Log2 nTbW + 1 (ii) wherein based on the width of the current block being greater than the height of the current block, the DC value is derived based on an equation as follow: dcVal = ∑ x ′ = 0 nTbW − 1 p x ′ − 1 − refIdx + nTbW ≫ 1 ≫ Log2 nTbW (iii) wherein based on the width of the current block being smaller than the height of the current block, the DC value is derived based on an equation as follow: dcVal = ∑ y ′ = 0 nTbH − 1 p − 1 − refIdx y ′ + nTbH ≫ 1 ≫ Log2 nTbH where dcVal represents the DC value and nTbW represents the width of the current block, nTbHs represents the height of the current block, refldx represents a value of a reference line index, and wherein the prediction related information includes information on the reference line index specifying the intra prediction reference line for the MRL. deriving an intra prediction type for a current block as Multi Reference Line intra prediction based on prediction related information, wherein the MRL is an intra prediction type based on an intra prediction reference line not adjacent to the current block; constructing an intra prediction mode candidate list including a DC intra prediction mode based on intra prediction modes of neighboring blocks of the current block; deriving an intra prediction mode of the current block based on the intra prediction mode candidate list; deriving a prediction sample of the current block based the intra prediction mode; deriving a residual sample of the current block based on residual information; and generating a reconstructed sample based on the prediction sample and the residual sample, characterized in that, based on the intra prediction mode of the current block being derived as the DC intra prediction mode, the prediction sample is derived as a DC value, wherein the DC value is derived based on a width of the current block and a height of the current block, (i) wherein based on the width of the current block being equal to the height of the current block, the DC value is derived based on an equation as follow: dcVal = ∑ x ′ = 0 nTbW − 1 p x ′ − 1 − refIdx + ∑ y ′ = 0 nTbH − 1 p − 1 − refIdx y ′ + nTbW ≫ Log2 nTbW + 1 (ii) wherein based on the width of the current block being greater than the height of the current block, the DC value is derived based on an equation as follow: dcVal = ∑ x ′ = 0 nTbW − 1 p x ′ − 1 − refIdx + nTbW ≫ 1 ≫ Log2 nTbW (iii) wherein based on the width of the current block being smaller than the height of the current block, the DC value is derived based on an equation as follow: dcVal = ∑ y ′ = 0 nTbH − 1 p − 1 − refIdx y ′ + nTbH ≫ 1 ≫ Log2 nTbH (i) wherein based on the width of the current block being equal to the height of the current block, the DC value is derived based on an equation as follow: dcVal = ∑ x ′ = 0 nTbW − 1 p x ′ − 1 − refIdx + ∑ y ′ = 0 nTbH − 1 p − 1 − refIdx y ′ + nTbW ≫ Log2 nTbW + 1 (ii) wherein based on the width of the current block being greater than the height of the current block, the DC value is derived based on an equation as follow: dcVal = ∑ x ′ = 0 nTbW − 1 p x ′ − 1 − refIdx + nTbW ≫ 1 ≫ Log2 nTbW (iii) wherein based on the width of the current block being smaller than the height of the current block, the DC value is derived based on an equation as follow: dcVal = ∑ y ′ = 0 nTbH − 1 p − 1 − refIdx y ′ + nTbH ≫ 1 ≫ Log2 nTbH where dcVal represents the DC value and nTbW represents the width of the current block, nTbHs represents the height of the current block, refldx represents a value of a reference line index, and wherein the prediction related information includes information on the reference line index specifying the intra prediction reference line for the MRL.