PURPOSE: To prevent a black color from being not correctly expressed and being a gray color or deformed in black levels by detecting a DC level of a front porch or a back porch, referencing a DC level and deciding a black level of RGB signals.

CONSTITUTION: A digital data subjected to A/D conversion is inputted to an input terminal 201 and the result is separated into a Y+DC data and a C data by line memories 203, 204 and a Y/C separator circuit 205. On the other hand, a signal whose frequency is 4×fsc is inputted to an input terminal 202, the C data enters latch circuits 207, 208, in which the data is latched in a prescribed timing. Then multipliers 209, 210 multiply the coefficient and the outputs of them are color difference signals (R-Y), (B-Y). Then a luminance signal Y+DC being an output of the Y/C separator circuit 205 enters a DC level detection circuit 217, in which a DC level of a back porch is detected and the content of the conversion table is decided based on the DC value. Thus, the color demodulation of RGB signals is attained, a black color is correctly expressed from any video equipment and a gray color forming or black level deformation is prevented.