In a recording medium selecting step, a recording medium on which an image is to be formed is selected from among a plurality of recording media having unique color tones. In a mode setting step, it is determined whether the image is to be output in a monochrome mode. In a signal converting step, a luminance signal corresponding to the image is converted into a density signal when the monochrome mode is set in the mode setting step. In a forming step, an image is formed on the recording medium selected in the recording medium selecting step according to the density signal. The signal converting step is controlled so that a color tone of an intermediate-density portion of the image formed on the recording medium is set to be near a*=A in the a*b* plane of the CIE-L*a*b* space, where A is a constant.