A method for adjusting a developing device including a magnetic roll for holding a two-component developer composed of a carrier and toner filled in a developing tank in a brush state, and a developing roll for developing an electrostatic latent image formed in an image carrier by holding toner conveyed from the magnetic roll, includes a bias voltage adjustment step for adjusting a conveyance amount of toner from the magnetic roll to the developing roll by adjusting bias voltages applied to the magnetic roll and the developing roll respectively, and a mixing ratio adjustment step for adjusting a conveyance amount of the toner by adjusting a mixing ratio of the two-component developer.