1. An image forming apparatus comprising: a rotatable image supporter having photoconductive properties; an optical image irradiating device configured to irradiate the image supporter with a light beam to form an electrostatic latent image on the image supporter; and a developing device including a developer, the developer including a toner and a magnetic carrier, and a developer supporter which bears the developer on a surface thereof and configured to feed the developer to a developing area between the image supporter and the developer supporter to develop the electrostatic latent image on the image supporter, wherein a ratio of a toner content in a first portion of the developer in the developing area relatively close to a surface of the image supporter to a toner content in a second portion of the developer in the developing area relatively close to the surface of the developer supporter is not less than about 0.6, measured when the developer contacts a continuous non-image area formed on the image supporter.