An image forming apparatus is provided. The image forming apparatus includes a photoconductor, an irradiator, a developing device including a developing roller, a development bias applying device, a toner supplying roller and a supply bias applying device, and a controller. The controller changes supply bias offset by reference to at least one of five parameters including photoconductor travel distance, operation temperature of the developing device, operation humidity of the developing device, continuous operating time of the developing device, and color information of image to be produced. After changing the supply bias offset, the controller forms plural toner patterns for image density adjustment on the photoconductor by changing the development bias, and sets the development bias to a certain bias based on the image densities of the plural toner patterns so that the image produced by the developing device has a targeted image density.