If all colors of a picture obtained by a first color picture device cannot be outputted from a second color picture device, a plurality of color data which are outside the gamut of the second color picture device and are inside the gamut of the first color picture device are converted so as to preserve correspondence regarding both the lightness value and the saturation value. Furthermore, if, since an equi-hue line has distortion, the tone of a color represented by color data is changed by conversion, the hue angle of the color data is rotated and converted. In picture data, color data for the color of paper or the color of a picture digitally generated have extreme values, and errors occur when the data are converted. In this case, data around the color data in a color conversion table are modified to obtain a desirable conversion result.