1. An image forming apparatus, comprising: a 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; a 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; wherein the parameter setting circuit selects one of a first mode and a second mode based on setting data, the first mode immediately changing the ratio of the reference value to the average printing rate or the average amount of toner consumption into the printing rate parameter value, and the second mode changing the ratio of the reference value to the average printing rate or the average amount of toner consumption in phases into the printing rate parameter values.