A small domain generation section generates a plurality of small domains in a multi-valued image transmitted from an intermediate tone correction section. Each domain is constituted of a plurality of pixels. Counting means counts a signal input value of each pixel. Weighting calculation means corrects each signal input value of the “pixels subjection to toner quantity equivalent value calculation” with reference to the signal input value of the small domain, and reads out a weighting coefficient from the weighting coefficient table based on the modified signal input value. The weighting calculation means multiplies the signal input value by the coefficient so as to convert the signal input value into a toner quantity equivalent value. Accumulating means calculates toner quantity equivalent values for all of the pixels of the multi-valued image. A total toner quantity equivalent value calculation section determines a total toner quantity equivalent value by accumulating all of the toner quantity equivalent values which are individually calculated each time an image is processed. When the total toner quantity equivalent value reaches a predetermined value, conditions for process control is set. On this account, the present invention achieves an image forming apparatus capable of accurate estimation of toner consumption quantity.