PURPOSE: To improve the compression rate by moving picture data represented by luminance and color difference signals to a part not in use in a 3-dimension space formed by axes of luminance and color difference so as to use an idle part.

CONSTITUTION: When range of values of Y, R-Y, B-Y signals inputted from an input terminal on each plane are taken in a 3-dimension space by the signals Y, R-Y, B-Y, the space is understood to be a convex polyhedron because of the linearity of a conversion equation. Thus, when each element of the signals is quantized independently, much part of a volume of a rectangular parallelepiped shown in an outer frame is not fully in use. Thus, bit reduction is attained by moving the remaining part of an octahedron to the part of the rectangular parallelepiped not in use. That is, the conversion table is so configured that a coordinate is moved to an idle part of the rectangular parallelepiped when either of the R-Y, B-Y is negative. Then the converted signal is inputted to an input terminal of an expansion part through a transmission line and given to the expansion conversion table.