An image decoding method performed by a decoding device, comprising: acquiring information on an intra-prediction mode of a current block from a received bit stream; deriving neighboring reference samples of the current block; deriving a first prediction value for a current sample of the current block based on the intra-prediction mode and the neighboring reference samples; deriving a second prediction value for the current sample based on adjacent samples of the current sample; and deriving a prediction value for the current sample based on the first prediction value and the second prediction value, wherein the image decoding method further comprising: generating virtual reference samples for deriving the second prediction value, wherein at least one of the adjacent samples is included in the virtual reference samples, wherein when a size of the current block is N x N and the coordinate of the upper left sample position of the current block is (0, 0), the virtual reference samples prediction value are derived based on the following equation, BR = A 2 N − 1 + L 2 N − 1 / 2 B x = L N + dif f B ⋅ x + 1, dif f B = BR − L N N + 1 R y = A N + dif f R ⋅ y + 1, dif f R = BR − A N N + 1 where BR represents the virtual reference sample of coordinate (N, N), Bx represents the virtual reference sample of coordinate (x, N), Ry represents the virtual reference sample of coordinate (N, y), Ax represents a neighboring reference sample of coordinate (x, -1), and Ly represents the neighboring reference sample of coordinate (-1, y), and wherein an adjacent sample which is not included in the virtual sample among the adjacent samples is located at the current block. acquiring information on an intra-prediction mode of a current block from a received bit stream; deriving neighboring reference samples of the current block; deriving a first prediction value for a current sample of the current block based on the intra-prediction mode and the neighboring reference samples; deriving a second prediction value for the current sample based on adjacent samples of the current sample; and deriving a prediction value for the current sample based on the first prediction value and the second prediction value, wherein the image decoding method further comprising: generating virtual reference samples for deriving the second prediction value, wherein at least one of the adjacent samples is included in the virtual reference samples, wherein when a size of the current block is N x N and the coordinate of the upper left sample position of the current block is (0, 0), the virtual reference samples prediction value are derived based on the following equation, BR = A 2 N − 1 + L 2 N − 1 / 2 B x = L N + dif f B ⋅ x + 1, dif f B = BR − L N N + 1 R y = A N + dif f R ⋅ y + 1, dif f R = BR − A N N + 1 where BR represents the virtual reference sample of coordinate (N, N), Bx represents the virtual reference sample of coordinate (x, N), Ry represents the virtual reference sample of coordinate (N, y), Ax represents a neighboring reference sample of coordinate (x, -1), and Ly represents the neighboring reference sample of coordinate (-1, y), and wherein an adjacent sample which is not included in the virtual sample among the adjacent samples is located at the current block. generating virtual reference samples for deriving the second prediction value, wherein at least one of the adjacent samples is included in the virtual reference samples, wherein when a size of the current block is N x N and the coordinate of the upper left sample position of the current block is (0, 0), the virtual reference samples prediction value are derived based on the following equation, BR = A 2 N − 1 + L 2 N − 1 / 2 B x = L N + dif f B ⋅ x + 1, dif f B = BR − L N N + 1 R y = A N + dif f R ⋅ y + 1, dif f R = BR − A N N + 1 where BR represents the virtual reference sample of coordinate (N, N), Bx represents the virtual reference sample of coordinate (x, N), Ry represents the virtual reference sample of coordinate (N, y), Ax represents a neighboring reference sample of coordinate (x, -1), and Ly represents the neighboring reference sample of coordinate (-1, y), and wherein an adjacent sample which is not included in the virtual sample among the adjacent samples is located at the current block.