Color image data composed of pixels arranged in a matrix wherein colors are expressed in bit rows is created from an acquired document file, and same-resolution binary image data and low-resolution binary image data composed of binary pixels arranged in a matrix are created based on the color image data. A first combining process is performed in which a lower bit value of a predetermined color of the pixels of the color image data is replaced with the value of the corresponding ones of the pixels of the same pixels, and a second combining process is performed in which a lower bit value of a color other than the bit replaced in the first combining process in the pixels of the first combined image data is replaced with the value of the corresponding ones of the pixels of the low-resolution binary image data.