As image processing apparatus includes a color converter 20 as a color converting device. The color converter 20 has, as main functional components for carrying out color conversion of halftone of a spot color, a spot color information acquiring unit 21 for acquiring spot color information from manuscript data, a spot color Lab acquiring unit 22 for acquiring L*a*b* values in an L*a*b* color space when the dot percentage of the spot color is 100%, a first calculating unit 23 for calculating halftone L*a*b* values, using the L*a*b* values at 100% dot percentage of the spot color acquired by the spot color Lab acquiring unit 22 and a dot percentage of the spot color in a printing target portion of the spot color, and a second calculating unit 24 for converting, using an output ICC profile, the halftone L*a*b* values calculated by the first calculating unit 23 into CMYK values.