1. An image forming apparatus comprising: a photoconductor to bear an electrostatic latent image thereon; an irradiator to irradiate the photoconductor to form the electrostatic latent image; at least one developing device including: a rotatable developing roller which is arranged so as to face the photoconductor while bearing thereon a toner serving as a one component developer to feed the toner to the photoconductor to develop the electrostatic latent image with the toner, thereby forming a toner image on the photoconductor; a development bias applying device to apply a development bias to the developing roller; a rotatable toner supplying roller to supply the toner to the developing roller; a supply bias applying device to apply a supply bias to the toner supplying roller; and at least one controller to control the development bias applying device and the supply bias applying device, wherein the at least one controller performs a development control including changing a supply bias offset, which is defined as a difference between the development bias and the supply bias, by reference to at least one of five parameters including photoconductor travel distance, operation temperature of the at least one developing device, operation humidity of the at least one developing device, continuous operating time of the at least one developing device, and color information of the toner image to be produced; then forming plural toner patterns for image density adjustment having different image densities on the photoconductor while changing the development bias, wherein forming the toner patterns for image density adjustment includes forming half-tone toner images by changing irradiation conditions under which the irradiator irradiates the photoconductor, and the development bias while fixing the supply bias offset and a background potential, which is defined as a difference between a surface potential of the photoconductor and the development bias; and setting the development bias to a certain bias based on the image densities of the plural toner patterns so that the toner image produced by the developing device has a targeted image density.