A decoding device includes a decoder that decodes information of a direction of intra-picture prediction of a target block and compression data of a residual signal, a prediction signal generator that generates an intra-picture prediction signal using the information of the direction and an previously reconstructed reference sample of an adjacent block, a residual signal restorator that restores a reconstructed residual signal of the target block, and a block storage that restores and stores a pixel signal of the target block. The prediction signal generator derives reference samples from a previously reconstructed block neighbouring the target block stored, selects two or more key reference samples, performs an interpolation process between the key reference samples for generating interpolated reference samples, and generates the intra-picture prediction signal by extrapolating the interpolated reference samples based on the direction of the intra-picture prediction.