A decoding device includes decoding means for decoding information of a direction of intra-picture prediction of a target block and compression data of a residual signal, prediction signal generation means for generating an intra-picture prediction signal using the information of the direction and an previously reconstructed reference sample of an adjacent block, residual signal restoration means for restoring a reconstructed residual signal of the target block, and block storage means for restoring and storing a pixel signal of the target block. The prediction signal generation means derives reference samples from an 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.