Even when an image signal is converted into an image signal with a limited amount of toner adhesion, the gradation is prevented from changing rapidly near the limiting value and therefore the image quality is prevented from deteriorating. An amount-of-adhesion converting circuit 301 converts image data for each color plane into first amount-of-toner-adhesion data. On the basis of a preset amount-of-toner-adhesion threshold value table, an amount-of-toner-adhesion reduction computing circuit 303 converts the first amount-of-toner-adhesion data for each color plane into second amount-of-toner-adhesion data with a limited amount of toner adhesion. Then, an image signal converting circuit 304 converts the second amount-of-toner-adhesion data into an image signal. In such a method, since parameters in the amount-of-toner-adhesion threshold value table are set arbitrarily, a rapid change in gradation can be suppressed near the limiting value.