1. An image data transformation apparatus, comprising: binarizing means for converting image data inputted thereto into binary image data associated with a plurality of different colors; binary image data buffer means for temporarily storing therein the binary image data; monochrome/color decision means for deciding based on the inputted image data whether a monochrome output or a color output is to be produced; and binary image data selection/output means for reading, for the monochrome output, from said binary image data buffer means, a data item of the binary image data associated with the plural colors and for reading therefrom, converted for the color output, at least two data items of the binary image associated with the plural colors, thereby outputting the attained data; wherein said monochrome/color decision means makes the decision based on the number of monochrome pixels and the number of color pixels in an overall area or a partial area of a frame of image of the input image data.