A development bias calculation and an electrifying bias calculation are executed in this order. In the development bias calculation, a plurality of toner images are formed as first patch images (PI1) while changing the development bias. An optimal development bias, which is necessary to obtain the target density, is determined based on densities of the first patch images (PI1). In the electrifying bias calculation, toner images are formed as second patch images (PI2) while changing the electrifying bias with the development bias fixed to the optimal development bias. An optimal electrifying bias, which is necessary to obtain the target density, is determined based on densities of the second patch images (PI2).