In case of a developing density correction based on a test image (patch image) density, normally, the developing density is carried out in a short period of time by correction (γ correction) of a developing bias and a grid voltage (S11). When a correction amount of the γ correction exceeds an ordinary range, the γ correction (S11) and adjustment of toner concentration (magnetic permeability reference value) (S5) are carried out in combination. Furthermore, when the toner concentration is changed, the setting of a correction reference of the γ correction (γ correction TBL) and correction timing of the γ correction are accordingly changed (S6, S7). Therefore, in the developing density correction based on the test image density in the developing device using binary developer, it is possible to carry out the developing density correction, which is carried out in a short time, and whose correction width is wide, while maintaining the accuracy of a developing density adjustment.