According to the present invention, in converting an interlaced signal in 4:2:0 format into an interlaced signal in 4:2:2 format, diagonal correlation is detected using a luminance signal and color-difference signals are also interpolated diagonally based on a result of the detection. As a result, the quality of the image represented by the color-difference signals can be improved and the signal can be converted into the 4:2:2 format with the block noise of the color-difference signals, which could be produced in an MPEG decoded image in the 4:2:0 format, reduced.