An image processing apparatus includes bus, adding portion, image processing portion, and output processing portion. In the case where document sheet data as a processing target is monochrome image data, the adding portion outputs, to the bus, color image data obtained by adding, to the monochrome image data, dummy image data pieces for a plurality of colors each corresponding to a predetermined processing unit amount which corresponds to a part of the monochrome image data. The image processing portion performs image processing per data amount equal to or smaller than the processing unit amount and outputs the resultant data to the bus. The output processing portion outputs image data pieces for a plurality of colors included in the color image data processed by the image processing and acquired from the bus, in accordance with synchronization signals inputted for the respective image data pieces for the plurality of colors.