Abstract: An intra-prediction unit generates an intra-prediction signal of an image. A subtraction unit calculates a first residual signal from a difference between an original signal of the image and the intra-prediction signal. A residual approximation signal creation unit creates a residual approximation signal by linearly approximating the first residual signal based on a DC component of the first residual signal. A subtraction unit calculates a second residual signal from a difference between the first residual signal and the residual approximation signal. An orthogonal transformation coefficient integration unit generates a third residual signal by synthesizing the DC component of the first residual signal and the second residual signal. A variable-length encoding unit performs variable-length encoding of the third residual signal.