1. An image forming apparatus, comprising: print mode setting circuit that sets a print mode to a normal print mode or an ecological print mode whose toner usage is less than a toner usage of the normal print mode; an average printing amount identifying circuit that identifies an average printing rate in a predetermined count of printed sheets or an average amount of toner consumption per sheet; parameter setting circuit that decreases a printing rate parameter value in the ecological print mode based on a ratio of a predetermined reference value to the identified average printing rate or the identified average amount of toner consumption; and an image processing circuit that multiplies tone values of a print image by the printing rate parameter value in the ecological print mode to reduce a printing rate of the print image.