PURPOSE: To improve the encoding efficiency by reducing a motion compensation unit so as to reduce a prediction error even when a sampling ratio of a luminance component and a chrominance component is large.

CONSTITUTION: A block conversion circuit 20 outputs a luminance component in the unit of blocks each comprising 8 picture elements ×8 picture elements and in the unit of 2×2 blocks and outputs a color difference component in the unit of (1/2) block comprising 4 picture elements ×8 picture elements. A C block conversion circuit 21 converts the color difference component into a component in the unit of blocks and a DCT circuit 4 implements DCT processing in the unit of blocks. On the other hand, a motion vector detection circuit 15 obtains a motion vector in the unit of four luminance blocks from an output of the block conversion circuit 20. A C block conversion circuit 22 converts local decode data in the unit of (1/2) block and gives the result to a motion compensation circuit 12, which implements motion compensation in the unit of (1/2) macro block. The unit of motion compensation is halved in comparison with a conventional device, the motion compensation accuracy is improved, the prediction error is reduced and the encoding efficiency is enhanced.