An image processing unit includes a discriminating unit for discriminating, for every data unit of an input image signal, whether the data unit is black-and-white image data or color image data; a black-and-white image processing unit for detecting at least one of pre-stored line segment patterns from the data unit, and generating color signals for a black-and-white image corresponding to the detected line segment pattern when the discriminating unit has discriminated that the data unit is black-and-white image data; and a color image processing unit for generating color signals for a color image when the discriminating unit has discriminated that the data unit is color image data or when the black-and-white image processing unit has failed to detect any of the pre-stored line segment patterns from the data unit.