A toner replenishing device of a toner consumption calculating portion calculates a toner consumption using a gray scale value of each pixel in a multiple-value image inputted from an image data input portion. The calculated toner consumption is corrected by a corrected consumption calculating portion, and is then accumulated by a cumulative consumption calculating portion, and thereby a cumulative toner consumption is calculated. A main replenishing amount control portion controls a replenishing roller driving motor so as to replenish a predetermined first replenishing amount of a toner to a developing tank when the cumulative toner consumption reaches a reference consumption or more. Further, an auxiliary replenishing amount control portion controls the replenishing roller driving motor so as to start to replenish a predetermined second replenishing amount of a toner to the developing tank when a toner concentration obtained by a toner concentration obtaining portion reaches less than a start concentration.