Patent ID: 8200109

Claim:
An image forming apparatus comprising: a rotatable image bearing member; a developing device for forming a developing portion at which an electrostatic latent image formed on said image bearing member is developed with toner; a toner supply device for supplying toner to said developing device; a density detecting portion for detecting a density of a control toner image formed on said image bearing member; a toner ratio detecting portion for detecting a toner ratio in a two component developer contained in said developing device; a transfer member for forming a transfer portion at which the toner image formed on said image bearing member is transferred onto a transfer medium; a charging member, disposed downstream of said developing portion and upstream of said transfer portion with respect to a rotational direction of said image bearing member, for electrically charging the toner image formed by said developing device by being supplied with a bias of an identical polarity to a charge polarity of the toner; an adjusting portion for adjusting an amount of the toner supplied from said toner supply device so that the toner ratio is not below a preset lower limit when the toner ratio detected by said toner ratio detecting portion reaches the lower limit; and a bias determining portion for determining, after the adjustment by said adjusting device, a condition of a bias to be applied to said charging member on the basis of a detection result of said density detecting portion with respect to the density of the control toner image formed on said image bearing member.