IMAGE FORMING APPARATUS FOR MEASURING REFERENCE VALUE OF TONER CONCENTRATION

A developing device of an image forming apparatus includes a casing, two-component developer accommodated in the casing, and a stirring screw for conveying and stirring the developer in the casing by rotation in a predetermined direction. The image forming apparatus includes a toner concentration sensor that outputs a value indicative of the toner concentration of the developer in the casing. The image forming apparatus determines whether or not the developer in the casing has become uniform, based on the amount of change per unit time of the output value of the toner concentration sensor, acquired after the stirring of the developer in the casing by the stirring screw is started. It is possible to appropriately measure the reference value of the toner concentration.